diff --git a/core/core.services.yml b/core/core.services.yml
index bffe0aa068e3e771f1fdcb966c5b9d079465f80d..b61ac2091ffc858becbd15076020beffa6146b09 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -165,6 +165,21 @@ services:
     calls:
       - [addSubscriber, ['@http_client_simpletest_subscriber']]
       - [setUserAgent, ['Drupal (+http://drupal.org/)']]
+  theme.negotiator:
+    class: Drupal\Core\Theme\ThemeNegotiator
+    arguments: ['@access_check.theme']
+    calls:
+      - [setRequest, ['@request']]
+  theme.negotiator.default:
+    class: Drupal\Core\Theme\DefaultNegotiator
+    arguments: ['@config.factory']
+    tags:
+      - { name: theme_negotiator, priority: -100 }
+  theme.negotiator.ajax_base_page:
+    class: Drupal\Core\Theme\AjaxBasePageNegotiator
+    arguments: ['@csrf_token', '@config.factory']
+    tags:
+      - { name: theme_negotiator, priority: 1000 }
   container.namespaces:
     class: ArrayObject
     arguments: [ '%container.namespaces%' ]
diff --git a/core/includes/ajax.inc b/core/includes/ajax.inc
index 7ef88c2c73cfc3e38e23202165b177ca1431ac52..5c4bb9968695517e928b1b19573edf0295837ccb 100644
--- a/core/includes/ajax.inc
+++ b/core/includes/ajax.inc
@@ -302,42 +302,6 @@ function ajax_render($commands = array()) {
   return drupal_json_encode($commands);
 }
 
-/**
- * Theme callback: Returns the correct theme for an Ajax request.
- *
- * Many different pages can invoke an Ajax request to system/ajax or another
- * generic Ajax path. It is almost always desired for an Ajax response to be
- * rendered using the same theme as the base page, because most themes are built
- * with the assumption that they control the entire page, so if the CSS for two
- * themes are both loaded for a given page, they may conflict with each other.
- * For example, Bartik is Drupal's default theme, and Seven is Drupal's default
- * administration theme. Depending on whether the "Use the administration theme
- * when editing or creating content" checkbox is checked, the node edit form may
- * be displayed in either theme, but the Ajax response to the Field module's
- * "Add another item" button should be rendered using the same theme as the rest
- * of the page. Therefore, system_menu() sets the 'theme callback' for
- * 'system/ajax' to this function, and it is recommended that modules
- * implementing other generic Ajax paths do the same.
- *
- * @see system_menu()
- * @see file_menu()
- */
-function ajax_base_page_theme() {
-  if (!empty($_POST['ajax_page_state']['theme']) && !empty($_POST['ajax_page_state']['theme_token'])) {
-    $theme = $_POST['ajax_page_state']['theme'];
-    $token = $_POST['ajax_page_state']['theme_token'];
-
-    // Prevent a request forgery from giving a person access to a theme they
-    // shouldn't be otherwise allowed to see. However, since everyone is allowed
-    // to see the default theme, token validation isn't required for that, and
-    // bypassing it allows most use-cases to work even when accessed from the
-    // page cache.
-    if ($theme === \Drupal::config('system.theme')->get('default') || drupal_valid_token($token, $theme)) {
-      return $theme;
-    }
-  }
-}
-
 /**
  * Converts the return value of a page callback into an Ajax commands array.
  *
diff --git a/core/includes/common.inc b/core/includes/common.inc
index c946eafdb2b5324847304b1e944e8fa10801aa4a..91956d477a40021d595aa83013da2678d7dd7c68 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -2273,7 +2273,7 @@ function drupal_get_js($scope = 'header', $javascript = NULL, $skip_alter = FALS
       global $theme_key;
       // Provide the page with information about the theme that's used, so that
       // a later AJAX request can be rendered using the same theme.
-      // @see ajax_base_page_theme()
+      // @see \Drupal\Core\Theme\AjaxBasePageNegotiator
       $setting['ajaxPageState']['theme'] = $theme_key;
       // Checks that the DB is available before filling theme_token.
       if (!defined('MAINTENANCE_MODE')) {
@@ -3090,7 +3090,6 @@ function _drupal_bootstrap_full($skip = FALSE) {
   // Let all modules take action before the menu system handles the request.
   // We do not want this while running update.php.
   if (!defined('MAINTENANCE_MODE') || MAINTENANCE_MODE != 'update') {
-    menu_set_custom_theme();
     drupal_theme_initialize();
   }
 }
diff --git a/core/includes/menu.inc b/core/includes/menu.inc
index b83c3238291456bea8539c7076be144fe335ca36..305b8f7b515c4404253ec408a4ef1b2e7d620305 100644
--- a/core/includes/menu.inc
+++ b/core/includes/menu.inc
@@ -463,8 +463,8 @@ function menu_set_item($path, $router_item) {
  *   menu_router table. The value corresponding to the key 'map' holds the
  *   loaded objects. The value corresponding to the key 'access' is TRUE if the
  *   current user can access this page. The values corresponding to the keys
- *   'title', 'page_arguments', 'access_arguments', and 'theme_arguments' will
- *   be filled in based on the database values and the objects loaded.
+ *   'title', 'page_arguments', and 'access_arguments', will be filled in based
+ *   on the database values and the objects loaded.
  */
 function menu_get_item($path = NULL, $router_item = NULL) {
   $router_items = &drupal_static(__FUNCTION__);
@@ -501,7 +501,6 @@ function menu_get_item($path = NULL, $router_item = NULL) {
       if ($router_item['access']) {
         $router_item['map'] = $map;
         $router_item['page_arguments'] = array_merge(menu_unserialize($router_item['page_arguments'], $map), array_slice($map, $router_item['number_parts']));
-        $router_item['theme_arguments'] = array_merge(menu_unserialize($router_item['theme_arguments'], $map), array_slice($map, $router_item['number_parts']));
       }
     }
     $router_items[$path] = $router_item;
@@ -1795,51 +1794,6 @@ function drupal_help_arg($arg = array()) {
   return $arg + array('', '', '', '', '', '', '', '', '', '', '', '');
 }
 
-/**
- * Gets the custom theme for the current page, if there is one.
- *
- * @param $initialize
- *   This parameter should only be used internally; it is set to TRUE in order
- *   to force the custom theme to be initialized for the current page request.
- *
- * @return
- *   The machine-readable name of the custom theme, if there is one.
- *
- * @see menu_set_custom_theme()
- */
-function menu_get_custom_theme($initialize = FALSE) {
-  $custom_theme = &drupal_static(__FUNCTION__);
-  // Skip this if the site is offline or being installed or updated, since the
-  // menu system may not be correctly initialized then.
-  if ($initialize && !_menu_site_is_offline(TRUE) && (!defined('MAINTENANCE_MODE') || (MAINTENANCE_MODE != 'update' && MAINTENANCE_MODE != 'install'))) {
-    // First allow modules to dynamically set a custom theme for the current
-    // page. Since we can only have one, the last module to return a valid
-    // theme takes precedence.
-    $custom_themes = array_filter(\Drupal::moduleHandler()->invokeAll('custom_theme'), 'drupal_theme_access');
-    if (!empty($custom_themes)) {
-      $custom_theme = array_pop($custom_themes);
-    }
-    // If there is a theme callback function for the current page, execute it.
-    // If this returns a valid theme, it will override any theme that was set
-    // by a hook_custom_theme() implementation above.
-    $router_item = menu_get_item();
-    if (!empty($router_item['access']) && !empty($router_item['theme_callback'])) {
-      $theme_name = call_user_func_array($router_item['theme_callback'], $router_item['theme_arguments']);
-      if (drupal_theme_access($theme_name)) {
-        $custom_theme = $theme_name;
-      }
-    }
-  }
-  return $custom_theme;
-}
-
-/**
- * Sets a custom theme for the current page, if there is one.
- */
-function menu_set_custom_theme() {
-  menu_get_custom_theme(TRUE);
-}
-
 /**
  * Returns an array containing the names of system-defined (default) menus.
  */
@@ -3071,13 +3025,6 @@ function _menu_router_build($callbacks, $save = FALSE) {
             }
           }
         }
-        // Same for theme callbacks.
-        if (!isset($item['theme callback']) && isset($parent['theme callback'])) {
-          $item['theme callback'] = $parent['theme callback'];
-          if (!isset($item['theme arguments']) && isset($parent['theme arguments'])) {
-            $item['theme arguments'] = $parent['theme arguments'];
-          }
-        }
         // Same for load arguments: if a loader doesn't have any explict
         // arguments, try to find arguments in the parent.
         if (!isset($item['load arguments'])) {
@@ -3114,8 +3061,6 @@ function _menu_router_build($callbacks, $save = FALSE) {
       'page callback' => '',
       'title arguments' => array(),
       'title callback' => 't',
-      'theme arguments' => array(),
-      'theme callback' => '',
       'description' => '',
       'description arguments' => array(),
       'description callback' => 't',
@@ -3180,8 +3125,6 @@ function _menu_router_save($menu, $masks) {
       'title',
       'title_callback',
       'title_arguments',
-      'theme_callback',
-      'theme_arguments',
       'type',
       'description',
       'description_callback',
@@ -3212,8 +3155,6 @@ function _menu_router_save($menu, $masks) {
       'title' => $item['title'],
       'title_callback' => $item['title callback'],
       'title_arguments' => ($item['title arguments'] ? serialize($item['title arguments']) : ''),
-      'theme_callback' => $item['theme callback'],
-      'theme_arguments' => serialize($item['theme arguments']),
       'type' => $item['type'],
       'description' => $item['description'],
       'description_callback' => $item['description callback'],
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index aaa3b74c5c5c83e033455982adf4bae48608b338..a83b7a5fc889ecb647a9578502040fcd0b0e17d0 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -94,14 +94,11 @@ function drupal_theme_initialize() {
   drupal_bootstrap(DRUPAL_BOOTSTRAP_DATABASE);
   $themes = list_themes();
 
-  // Only select the user selected theme if it is available in the
-  // list of themes that can be accessed.
-  $theme = !empty($user->theme) && drupal_theme_access($user->theme) ? $user->theme : \Drupal::config('system.theme')->get('default');
-
-  // Allow modules to override the theme. Validation has already been performed
-  // inside menu_get_custom_theme(), so we do not need to check it again here.
-  $custom_theme = menu_get_custom_theme();
-  $theme = !empty($custom_theme) ? $custom_theme : $theme;
+  // @todo Let the theme.negotiator listen to the kernel request event.
+  // Determine the active theme for the theme negotiator service. This includes
+  // the default theme as well as really specific ones like the ajax base theme.
+  $request = \Drupal::request();
+  $theme = \Drupal::service('theme.negotiator')->determineActiveTheme($request) ?: 'stark';
 
   // Store the identifier for retrieving theme settings with.
   $theme_key = $theme;
@@ -114,9 +111,6 @@ function drupal_theme_initialize() {
     $base_theme[] = $themes[$ancestor];
   }
   _drupal_theme_initialize($themes[$theme], array_reverse($base_theme));
-
-  // Themes can have alter functions, so reset the drupal_alter() cache.
-  drupal_static_reset('drupal_alter');
 }
 
 /**
diff --git a/core/lib/Drupal/Core/CoreServiceProvider.php b/core/lib/Drupal/Core/CoreServiceProvider.php
index 8a525c9abdbcd96eaa0d80cfe5642ba41408399c..098b1653aa770df172baf485241ec29f52149a1c 100644
--- a/core/lib/Drupal/Core/CoreServiceProvider.php
+++ b/core/lib/Drupal/Core/CoreServiceProvider.php
@@ -23,6 +23,7 @@
 use Drupal\Core\DependencyInjection\Compiler\RegisterBreadcrumbBuilderPass;
 use Drupal\Core\DependencyInjection\Compiler\RegisterAuthenticationPass;
 use Drupal\Core\DependencyInjection\Compiler\RegisterTwigExtensionsPass;
+use Drupal\Core\Theme\ThemeNegotiatorPass;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\DependencyInjection\Reference;
 use Symfony\Component\DependencyInjection\Definition;
@@ -71,6 +72,9 @@ public function register(ContainerBuilder $container) {
     // Add the compiler pass that will process the tagged breadcrumb builder
     // services.
     $container->addCompilerPass(new RegisterBreadcrumbBuilderPass());
+    // Add the compiler pass that will process the tagged theme negotiator
+    // service.
+    $container->addCompilerPass(new ThemeNegotiatorPass());
     // Add the compiler pass that lets service providers modify existing
     // service definitions.
     $container->addCompilerPass(new ModifyServiceDefinitionsPass());
diff --git a/core/lib/Drupal/Core/EventSubscriber/LegacyRequestSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/LegacyRequestSubscriber.php
index 39828ef352f73e0c4b44e5ca476d9bddbd858374..aa7774e90ffab7696c570ecfb70d1d176199ba48 100644
--- a/core/lib/Drupal/Core/EventSubscriber/LegacyRequestSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/LegacyRequestSubscriber.php
@@ -27,9 +27,6 @@ class LegacyRequestSubscriber implements EventSubscriberInterface {
    */
   public function onKernelRequestLegacy(GetResponseEvent $event) {
     if ($event->getRequestType() == HttpKernelInterface::MASTER_REQUEST) {
-      menu_set_custom_theme();
-      drupal_theme_initialize();
-
       // Tell Drupal it is now fully bootstrapped (for the benefit of code that
       // calls drupal_get_bootstrap_phase()), but without having
       // _drupal_bootstrap_full() do anything, since we've already done the
@@ -39,6 +36,18 @@ public function onKernelRequestLegacy(GetResponseEvent $event) {
     }
   }
 
+  /**
+   * Initializes the theme system after the routing system.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
+   *   The Event to process.
+   */
+  public function onKernelRequestLegacyAfterRouting(GetResponseEvent $event) {
+    if ($event->getRequestType() == HttpKernelInterface::MASTER_REQUEST) {
+      drupal_theme_initialize();
+    }
+  }
+
   /**
    * Registers the methods in this class that should be listeners.
    *
@@ -47,6 +56,8 @@ public function onKernelRequestLegacy(GetResponseEvent $event) {
    */
   static function getSubscribedEvents() {
     $events[KernelEvents::REQUEST][] = array('onKernelRequestLegacy', 90);
+    // Initialize the theme system after the routing system.
+    $events[KernelEvents::REQUEST][] = array('onKernelRequestLegacyAfterRouting', 30);
 
     return $events;
   }
diff --git a/core/lib/Drupal/Core/Theme/AjaxBasePageNegotiator.php b/core/lib/Drupal/Core/Theme/AjaxBasePageNegotiator.php
new file mode 100644
index 0000000000000000000000000000000000000000..4066b163ca4e973d8b66ee3e9d4d47eb714903a2
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/AjaxBasePageNegotiator.php
@@ -0,0 +1,90 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Theme\AjaxBasePageNegotiator.
+ */
+
+namespace Drupal\Core\Theme;
+
+use Drupal\Core\Access\CsrfTokenGenerator;
+use Drupal\Core\Config\ConfigFactory;
+use Symfony\Cmf\Component\Routing\RouteObjectInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Defines a theme negotiator that deals with the active theme on ajax requests.
+ *
+ * Many different pages can invoke an Ajax request to system/ajax or another
+ * generic Ajax path. It is almost always desired for an Ajax response to be
+ * rendered using the same theme as the base page, because most themes are built
+ * with the assumption that they control the entire page, so if the CSS for two
+ * themes are both loaded for a given page, they may conflict with each other.
+ * For example, Bartik is Drupal's default theme, and Seven is Drupal's default
+ * administration theme. Depending on whether the "Use the administration theme
+ * when editing or creating content" checkbox is checked, the node edit form may
+ * be displayed in either theme, but the Ajax response to the Field module's
+ * "Add another item" button should be rendered using the same theme as the rest
+ * of the page.
+ *
+ * Therefore specify '_theme: ajax_base_page' as part of the router options.
+ */
+class AjaxBasePageNegotiator implements ThemeNegotiatorInterface {
+
+  /**
+   * The CSRF token generator.
+   *
+   * @var \Drupal\Core\Access\CsrfTokenGenerator
+   */
+  protected $csrfGenerator;
+
+  /**
+   * The config factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactory
+   */
+  protected $configFactory;
+
+  /**
+   * Constructs a new AjaxBasePageNegotiator.
+   *
+   * @param \Drupal\Core\Access\CsrfTokenGenerator $token_generator
+   *   The CSRF token generator.
+   * @param \Drupal\Core\Config\ConfigFactory $config_factory
+   *   The config factory.
+   */
+  public function __construct(CsrfTokenGenerator $token_generator, ConfigFactory $config_factory) {
+    $this->csrfGenerator = $token_generator;
+    $this->configFactory = $config_factory;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(Request $request) {
+    // Check whether the route was configured to use the base page theme.
+    return ($route = $request->attributes->get(RouteObjectInterface::ROUTE_OBJECT))
+      && $route->hasOption('_theme')
+      && $route->getOption('_theme') == 'ajax_base_page';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(Request $request) {
+    if (($ajax_page_state = $request->request->get('ajax_page_state'))  && !empty($ajax_page_state['theme']) && !empty($ajax_page_state['theme_token'])) {
+      $theme = $ajax_page_state['theme'];
+      $token = $ajax_page_state['theme_token'];
+
+      // Prevent a request forgery from giving a person access to a theme they
+      // shouldn't be otherwise allowed to see. However, since everyone is
+      // allowed to see the default theme, token validation isn't required for
+      // that, and bypassing it allows most use-cases to work even when accessed
+      // from the page cache.
+      if ($theme === $this->configFactory->get('system.theme')->get('default') || $this->csrfGenerator->validate($token, $theme)) {
+        return $theme;
+      }
+    }
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Theme/DefaultNegotiator.php b/core/lib/Drupal/Core/Theme/DefaultNegotiator.php
new file mode 100644
index 0000000000000000000000000000000000000000..b08882d132f13079630147ef7cbf173fa8c32901
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/DefaultNegotiator.php
@@ -0,0 +1,49 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Theme\DefaultNegotiator.
+ */
+
+namespace Drupal\Core\Theme;
+
+use Drupal\Core\Config\ConfigFactory;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Determines the default theme of the site.
+ */
+class DefaultNegotiator implements ThemeNegotiatorInterface {
+
+  /**
+   * The system theme config object.
+   *
+   * @var \Drupal\Core\Config\Config
+   */
+  protected $config;
+
+  /**
+   * Constructs a DefaultNegotiator object.
+   *
+   * @param \Drupal\Core\Config\ConfigFactory $config_factory
+   *   The config factory.
+   */
+  public function __construct(ConfigFactory $config_factory) {
+    $this->config = $config_factory->get('system.theme');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(Request $request) {
+    return TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(Request $request) {
+    return $this->config->get('default');
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Theme/ThemeNegotiator.php b/core/lib/Drupal/Core/Theme/ThemeNegotiator.php
new file mode 100644
index 0000000000000000000000000000000000000000..51f3e67635c97b4aa65953c8f6e01b2f4866fea7
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/ThemeNegotiator.php
@@ -0,0 +1,142 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Theme\ThemeNegotiator.
+ */
+
+namespace Drupal\Core\Theme;
+
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Provides a class which determines the active theme of the page.
+ *
+ * It therefore uses ThemeNegotiatorInterface objects which are passed in
+ * using the 'theme_negotiator' tag.
+ *
+ * @see \Drupal\Core\Theme\ThemeNegotiatorPass
+ * @see \Drupal\Core\Theme\ThemeNegotiatorInterface
+ */
+class ThemeNegotiator implements ThemeNegotiatorInterface {
+
+  /**
+   * Holds arrays of theme negotiators, keyed by priority.
+   *
+   * @var array
+   */
+  protected $negotiators = array();
+
+  /**
+   * Holds the array of theme negotiators sorted by priority.
+   *
+   * Set to NULL if the array needs to be re-calculated.
+   *
+   * @var array|NULL
+   */
+  protected $sortedNegotiators;
+
+  /**
+   * The current request.
+   *
+   * @var \Symfony\Component\HttpFoundation\Request
+   */
+  protected $request;
+
+  /**
+   * The access checker for themes.
+   *
+   * @var \Drupal\Core\Theme\ThemeAccessCheck
+   */
+  protected $themeAccess;
+
+  /**
+   * Constructs a new ThemeNegotiator.
+   *
+   * @param \Drupal\Core\Theme\ThemeAccessCheck $theme_access
+   *   The access checker for themes.
+   */
+  public function __construct(ThemeAccessCheck $theme_access) {
+    $this->themeAccess = $theme_access;
+  }
+
+  /**
+   * Sets the request object to use.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The request object.
+   */
+  public function setRequest(Request $request) {
+    $this->request = $request;
+  }
+
+  /**
+   * Adds a active theme negotiation service.
+   *
+   * @param \Drupal\Core\Theme\ThemeNegotiatorInterface $negotiator
+   *   The theme negotiator to add.
+   * @param int $priority
+   *   Priority of the theme negotiator.
+   */
+  public function addNegotiator(ThemeNegotiatorInterface $negotiator, $priority) {
+    $this->negotiators[$priority][] = $negotiator;
+    // Force the negotiators to be re-sorted.
+    $this->sortedNegotiators = NULL;
+  }
+
+  /**
+   * Returns the sorted array of theme negotiators.
+   *
+   * @return array|\Drupal\Core\Theme\ThemeNegotiatorInterface[]
+   *   An array of theme negotiator objects.
+   */
+  protected function getSortedNegotiators() {
+    if (!isset($this->sortedNegotiators)) {
+      // Sort the negotiators according to priority.
+      krsort($this->negotiators);
+      // Merge nested negotiators from $this->negotiators into
+      // $this->sortedNegotiators.
+      $this->sortedNegotiators = array();
+      foreach ($this->negotiators as $builders) {
+        $this->sortedNegotiators = array_merge($this->sortedNegotiators, $builders);
+      }
+    }
+    return $this->sortedNegotiators;
+  }
+
+  /**
+   * Get the current active theme.
+   *
+   * @return string
+   *   The current active string.
+   */
+  public function getActiveTheme() {
+    if (!$this->request->attributes->has('_theme_active')) {
+      $this->determineActiveTheme($this->request);
+    }
+    return $this->request->attributes->get('_theme_active');
+  }
+
+  /**
+   * {@inheritdoc
+   */
+  public function applies(Request $request) {
+    return TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(Request $request) {
+    foreach ($this->getSortedNegotiators() as $negotiator) {
+      if ($negotiator->applies($request)) {
+        $theme = $negotiator->determineActiveTheme($request);
+        if ($theme !== NULL && $this->themeAccess->checkAccess($theme)) {
+          $request->attributes->set('_theme_active', $theme);
+          return $request->attributes->get('_theme_active');
+        }
+      }
+    }
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Theme/ThemeNegotiatorInterface.php b/core/lib/Drupal/Core/Theme/ThemeNegotiatorInterface.php
new file mode 100644
index 0000000000000000000000000000000000000000..e0631c2cf9fb58b64f62865fed61a1c46ed1652f
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/ThemeNegotiatorInterface.php
@@ -0,0 +1,53 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Theme\ThemeNegotiatorInterface.
+ */
+
+namespace Drupal\Core\Theme;
+
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Defines an interface for classes which determine the active theme.
+ *
+ * To set the active theme, create a new service tagged with 'theme_negotiator'
+ * (see user.services.yml for an example). The only method this service needs
+ * to implement is determineActiveTheme. Return the name of the theme, or NULL
+ * if other negotiators like the configured default one should kick in instead.
+ *
+ * If you are setting a theme which is closely tied to the functionality of a
+ * particular page or set of pages (such that the page might not function
+ * correctly if a different theme is used), make sure to set the priority on
+ * the service to a high number so that it is not accidentally overridden by
+ * other theme negotiators. By convention, a priority of "1000" is used in
+ * these cases; see \Drupal\Core\Theme\AjaxBasePageNegotiator and
+ * core.services.yml for an example.
+ */
+interface ThemeNegotiatorInterface {
+
+  /**
+   * Whether this theme negotiator should be used to set the theme.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The current request object.
+   *
+   * @return bool
+   *   TRUE if this negotiator should be used or FALSE to let other negotiators
+   *   decide.
+   */
+  public function applies(Request $request);
+
+  /**
+   * Determine the active theme for the request.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The active request of the site.
+   *
+   * @return string|null
+   *   Returns the active theme name, else return NULL.
+   */
+  public function determineActiveTheme(Request $request);
+
+}
diff --git a/core/lib/Drupal/Core/Theme/ThemeNegotiatorPass.php b/core/lib/Drupal/Core/Theme/ThemeNegotiatorPass.php
new file mode 100644
index 0000000000000000000000000000000000000000..7f1469295944498305c77fb8be13f94cc34088a4
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/ThemeNegotiatorPass.php
@@ -0,0 +1,36 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Theme\ThemeNegotiatorPass.
+ */
+
+namespace Drupal\Core\Theme;
+
+use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
+use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\DependencyInjection\Reference;
+
+/**
+ * Adds services to the theme negotiator service.
+ *
+ * @see \Drupal\Core\Theme\ThemeNegotiator
+ * @see \Drupal\Core\Theme\ThemeNegotiatorInterfa
+ */
+class ThemeNegotiatorPass implements CompilerPassInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function process(ContainerBuilder $container) {
+    if (!$container->hasDefinition('theme.negotiator')) {
+      return;
+    }
+    $manager = $container->getDefinition('theme.negotiator');
+    foreach ($container->findTaggedServiceIds('theme_negotiator') as $id => $attributes) {
+      $priority = isset($attributes[0]['priority']) ? $attributes[0]['priority'] : 0;
+      $manager->addMethodCall('addNegotiator', array(new Reference($id), $priority));
+    }
+  }
+
+}
diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index ad9dcae566da4acc9a08e07b76d506b9f592d869..6ecde951719b7d4d037f9d74fbc5616b3ecd0c84 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -382,7 +382,7 @@ Drupal.ajax.prototype.beforeSerialize = function (element, options) {
 
   // Allow Drupal to return new JavaScript and CSS files to load without
   // returning the ones already loaded.
-  // @see ajax_base_page_theme()
+  // @see \Drupal\Core\Theme\AjaxBasePageNegotiator
   // @see drupal_get_css()
   // @see drupal_get_js()
   var pageState = drupalSettings.ajaxPageState;
diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index 33a5946458c8da2754c32df5ce147030e1fe77d5..4570982a2ab6513d7cce899cffcfc95ce72c0007 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -113,41 +113,9 @@ function block_menu() {
     'type' => MENU_VISIBLE_IN_BREADCRUMB,
     'route_name' => 'block.admin_add',
   );
-  // Block administration is tied to the theme and plugin definition so
-  // that the plugin can appropriately attach to this URL structure.
-  // @todo D8: Use dynamic % arguments instead of static, hard-coded theme names
-  //   and plugin IDs to decouple the routes from these dependencies.
-  // @see http://drupal.org/node/1067408
-  foreach (list_themes() as $key => $theme) {
-    $items["admin/structure/block/demo/$key"] = array(
-      'route_name' => 'block.admin_demo',
-      'type' => MENU_CALLBACK,
-      'theme callback' => '_block_custom_theme',
-      'theme arguments' => array($key),
-    );
-  }
   return $items;
 }
 
-/**
- * Theme callback: Uses the theme specified in the parameter.
- *
- * @param $theme
- *   The theme whose blocks are being configured. If not set, the default theme
- *   is assumed.
- *
- * @return
- *   The theme that should be used for the block configuration page, or NULL
- *   to indicate that the default theme should be used.
- *
- * @see block_menu()
- */
-function _block_custom_theme($theme = NULL) {
-  // We return exactly what was passed in, to guarantee that the page will
-  // always be displayed using the theme whose blocks are being configured.
-  return $theme;
-}
-
 /**
  * Implements hook_page_build().
  *
diff --git a/core/modules/block/block.services.yml b/core/modules/block/block.services.yml
index b6bf1f41d373427fe61239426e9ceb29a5956aeb..34293d936929d702dac380647a3ade50e58ae381 100644
--- a/core/modules/block/block.services.yml
+++ b/core/modules/block/block.services.yml
@@ -9,3 +9,7 @@ services:
     factory_method: get
     factory_service: cache_factory
     arguments: [block]
+  theme.negotiator.block.admin_demo:
+    class: Drupal\block\Theme\AdminDemoNegotiator
+    tags:
+      - { name: theme_negotiator, priority: 1000 }
diff --git a/core/modules/block/lib/Drupal/block/Theme/AdminDemoNegotiator.php b/core/modules/block/lib/Drupal/block/Theme/AdminDemoNegotiator.php
new file mode 100644
index 0000000000000000000000000000000000000000..d175b91dd970a09ef06155a37e277f344958ec79
--- /dev/null
+++ b/core/modules/block/lib/Drupal/block/Theme/AdminDemoNegotiator.php
@@ -0,0 +1,35 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\block\Theme\AdminDemoNegotiator.
+ */
+
+namespace Drupal\block\Theme;
+
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
+use Symfony\Cmf\Component\Routing\RouteObjectInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Negotiates the theme for the block admin demo page via the URL.
+ */
+class AdminDemoNegotiator implements ThemeNegotiatorInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(Request $request) {
+    return $request->attributes->get(RouteObjectInterface::ROUTE_NAME) == 'block.admin_demo';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(Request $request) {
+    // We return exactly what was passed in, to guarantee that the page will
+    // always be displayed using the theme whose blocks are being configured.
+    return $request->attributes->get('theme');
+  }
+
+}
diff --git a/core/modules/content_translation/content_translation.module b/core/modules/content_translation/content_translation.module
index 73dcca9325bfb156337be4cbcb49fe67861d9b73..0d476cbb885a9461608d39ca26f25ef110ccbf11 100644
--- a/core/modules/content_translation/content_translation.module
+++ b/core/modules/content_translation/content_translation.module
@@ -168,7 +168,7 @@ function content_translation_menu() {
     if (content_translation_enabled($entity_type)) {
       $path = _content_translation_link_to_router_path($entity_type, $info['links']['canonical']);
       $entity_position = count(explode('/', $path)) - 1;
-      $keys = array_flip(array('theme_callback', 'theme_arguments', 'load_arguments'));
+      $keys = array_flip(array('load_arguments'));
       $menu_info = array_intersect_key($info['translation']['content_translation'], $keys) + array('file' => 'content_translation.pages.inc');
       $item = array();
 
diff --git a/core/modules/contextual/contextual.module b/core/modules/contextual/contextual.module
index d9d20b58fe44153ce8798edd1b81e6ae7782a537..a465e4767adf8c125e13159fda72c53f64685322 100644
--- a/core/modules/contextual/contextual.module
+++ b/core/modules/contextual/contextual.module
@@ -8,21 +8,6 @@
 use Drupal\Component\Utility\Url;
 use Drupal\Core\Template\Attribute;
 
-/**
- * Implements hook_menu().
- */
-function contextual_menu() {
-  // @todo Remove this menu item in http://drupal.org/node/1954892 when theme
-  //   callbacks are replaced with something else.
-  $items['contextual/render'] = array(
-    'route_name' => 'contextual.render',
-    'theme callback' => 'ajax_base_page_theme',
-    'type' => MENU_CALLBACK,
-  );
-
-  return $items;
-}
-
 /**
  * Implements hook_toolbar().
  */
diff --git a/core/modules/contextual/contextual.routing.yml b/core/modules/contextual/contextual.routing.yml
index 8ab2f2887bf5eeff1e15d5b05967bd7af585e845..b44d61920ea8f9c15370038d305fce1a6695c601 100644
--- a/core/modules/contextual/contextual.routing.yml
+++ b/core/modules/contextual/contextual.routing.yml
@@ -2,5 +2,7 @@ contextual.render:
   path: '/contextual/render'
   defaults:
     _controller: '\Drupal\contextual\ContextualController::render'
+  options:
+    _theme: ajax_base_page
   requirements:
     _permission: 'access contextual links'
diff --git a/core/modules/edit/edit.module b/core/modules/edit/edit.module
index b7de2d8195d609a4b39640665bef73c914696161..bc7b314c18004c154be2aa61b412df55c9d7cb42 100644
--- a/core/modules/edit/edit.module
+++ b/core/modules/edit/edit.module
@@ -14,26 +14,6 @@
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Entity\Display\EntityViewDisplayInterface;
 
-/**
- * Implements hook_menu().
- */
-function edit_menu() {
-  // @todo Remove these menu items in http://drupal.org/node/1954892 when theme
-  //   callbacks are replaced with something else.
-  $items['edit/metadata'] = array(
-    'route_name' => 'edit.metadata',
-    'theme callback' => 'ajax_base_page_theme',
-    'type' => MENU_CALLBACK,
-  );
-  $items['edit/form/%/%/%/%/%'] = array(
-    'route_name' => 'edit.field_form',
-    'theme callback' => 'ajax_base_page_theme',
-    'type' => MENU_CALLBACK,
-  );
-
-  return $items;
-}
-
 /**
  * Implements hook_permission().
  */
diff --git a/core/modules/edit/edit.routing.yml b/core/modules/edit/edit.routing.yml
index 41acaacc090a3d4d00b32dbb5ba3282c7574815c..31f6c62f2f1e97ec1012f5aadc7e9101806793e6 100644
--- a/core/modules/edit/edit.routing.yml
+++ b/core/modules/edit/edit.routing.yml
@@ -2,6 +2,8 @@ edit.metadata:
   path: '/edit/metadata'
   defaults:
     _controller: '\Drupal\edit\EditController::metadata'
+  options:
+    _theme: ajax_base_page
   requirements:
     _permission: 'access in-place editing'
 
@@ -16,6 +18,9 @@ edit.field_form:
   path: '/edit/form/{entity_type}/{entity}/{field_name}/{langcode}/{view_mode_id}'
   defaults:
     _controller: '\Drupal\edit\EditController::fieldForm'
+  options:
+    _access_mode: 'ALL'
+    _theme: ajax_base_page
   requirements:
     _permission: 'access in-place editing'
     _access_edit_entity_field: 'TRUE'
diff --git a/core/modules/editor/editor.module b/core/modules/editor/editor.module
index 0222b3083a5c897948f792c1a8632dc8823cb8c6..1234099b83dd64f7d413f2a9fad2520898bc2b40 100644
--- a/core/modules/editor/editor.module
+++ b/core/modules/editor/editor.module
@@ -138,21 +138,6 @@ function editor_library_info() {
   return $libraries;
 }
 
-/**
- * Implements hook_menu().
- */
-function editor_menu() {
-  // @todo Remove this menu item in http://drupal.org/node/1954892 when theme
-  //   callbacks are replaced with something else.
-  $items['editor/%/%/%/%/%'] = array(
-    'route_name' => 'editor.field_untransformed_text',
-    'theme callback' => 'ajax_base_page_theme',
-    'type' => MENU_CALLBACK,
-  );
-
-  return $items;
-}
-
 /**
  * Implements hook_form_FORM_ID_alter().
  */
diff --git a/core/modules/editor/editor.routing.yml b/core/modules/editor/editor.routing.yml
index 3308dd07efc44e94d49379a14c2f23b8ff195272..bf9d3607c52d26dce28d6bc5579932dbc51ad49e 100644
--- a/core/modules/editor/editor.routing.yml
+++ b/core/modules/editor/editor.routing.yml
@@ -2,6 +2,8 @@ editor.field_untransformed_text:
   path: '/editor/{entity_type}/{entity}/{field_name}/{langcode}/{view_mode_id}'
   defaults:
     _controller: '\Drupal\editor\EditorController::getUntransformedText'
+  options:
+    _theme: ajax_base_page
   requirements:
     _permission: 'access in-place editing'
     _access_edit_entity_field: 'TRUE'
diff --git a/core/modules/file/file.module b/core/modules/file/file.module
index 8c8519532295e62be78c8b35a285c0906003e182..40418869f4b419cc76e51a51ffc22515fd719631 100644
--- a/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -41,21 +41,6 @@ function file_help($path, $arg) {
   }
 }
 
-/**
- * Implements hook_menu().
- */
-function file_menu() {
-  $items = array();
-
-  $items['file/ajax'] = array(
-    'route_name' => 'file.ajax_upload',
-    'theme callback' => 'ajax_base_page_theme',
-    'type' => MENU_CALLBACK,
-  );
-
-  return $items;
-}
-
 /**
  * Implements hook_element_info().
  *
diff --git a/core/modules/file/file.routing.yml b/core/modules/file/file.routing.yml
index 8bf971c9a21e2ebacc36c54b399e118fbd0da60f..d9d4efa0aff61727f0fea37e414b384e606cba06 100644
--- a/core/modules/file/file.routing.yml
+++ b/core/modules/file/file.routing.yml
@@ -2,6 +2,8 @@ file.ajax_upload:
   path: '/file/ajax'
   defaults:
     _controller: '\Drupal\file\Controller\FileWidgetAjaxController::upload'
+  options:
+    _theme: ajax_base_page
   requirements:
     _permission: 'access content'
 
diff --git a/core/modules/search/lib/Drupal/search/Tests/SearchPageOverrideTest.php b/core/modules/search/lib/Drupal/search/Tests/SearchPageOverrideTest.php
index ab3f9135c86638a241f14d2e937c103dc113fdaa..101b745395f7d35e4ae9cd439136cf1c5cf36248 100644
--- a/core/modules/search/lib/Drupal/search/Tests/SearchPageOverrideTest.php
+++ b/core/modules/search/lib/Drupal/search/Tests/SearchPageOverrideTest.php
@@ -38,7 +38,7 @@ function setUp() {
 
     // Enable the extra type module for searching.
     \Drupal::config('search.settings')->set('active_plugins', array('node_search', 'user_search', 'search_extra_type_search'))->save();
-    \Drupal::state()->set('menu_rebuild_needed', TRUE);
+    \Drupal::service('router.builder')->rebuild();
   }
 
   function testSearchPageHook() {
diff --git a/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
index 832ca6428b9ac33f64a9ef3f81a241be579da605..10d56e265deb52956a3e28212baf1c77ad50c196 100644
--- a/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
+++ b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
@@ -159,6 +159,7 @@ public function testShortcutLinkDelete() {
    */
   public function testNoShortcutLink() {
     // Change to a theme that displays shortcuts.
+    theme_enable(array('seven'));
     \Drupal::config('system.theme')
       ->set('default', 'seven')
       ->save();
@@ -171,8 +172,8 @@ public function testNoShortcutLink() {
     $this->assertNoRaw('add-shortcut', 'Add to shortcuts link was not shown on a page the user does not have access to.');
 
     // Verify that the testing mechanism works by verifying the shortcut
-    // link appears on admin/content/node.
-    $this->drupalGet('admin/content');
+    // link appears on admin/people.
+    $this->drupalGet('admin/people');
     $this->assertRaw('remove-shortcut', 'Remove from shortcuts link was shown on a page the user does have access to.');
   }
 
diff --git a/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutTestBase.php b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutTestBase.php
index 64e537068adacae7a3a268fe29eaa17d2159dd00..58b0313750440fe58a684fc165e775b4928fab17 100644
--- a/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutTestBase.php
+++ b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutTestBase.php
@@ -71,7 +71,7 @@ function setUp() {
     }
 
     // Create users.
-    $this->admin_user = $this->drupalCreateUser(array('access toolbar', 'administer shortcuts', 'view the administration theme', 'create article content', 'create page content', 'access content overview'));
+    $this->admin_user = $this->drupalCreateUser(array('access toolbar', 'administer shortcuts', 'view the administration theme', 'create article content', 'create page content', 'access content overview', 'administer users'));
     $this->shortcut_user = $this->drupalCreateUser(array('customize shortcut links', 'switch shortcut sets'));
 
     // Create a node.
diff --git a/core/modules/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php b/core/modules/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php
index 71cf0c4d771354404724cdee79ec788b45d08c78..ccc327dcba383bfdf82f4b525909a17b6adf1c71 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php
@@ -481,18 +481,9 @@ public function testThemeIntegration() {
     $this->initializeTestThemeConfiguration();
     $this->doTestThemeCallbackFakeTheme();
 
-    $this->initializeTestThemeConfiguration();
-    $this->doTestHookCustomTheme();
-
-    $this->initializeTestThemeConfiguration();
-    $this->doTestThemeCallbackHookCustomTheme();
-
     $this->initializeTestThemeConfiguration();
     $this->doTestThemeCallbackAdministrative();
 
-    $this->initializeTestThemeConfiguration();
-    $this->doTestThemeCallbackInheritance();
-
     $this->initializeTestThemeConfiguration();
     $this->doTestThemeCallbackNoThemeRequested();
 
@@ -516,27 +507,17 @@ protected function initializeTestThemeConfiguration() {
   }
 
   /**
-   * Test the theme callback when it is set to use an administrative theme.
+   * Test the theme negotiation when it is set to use an administrative theme.
    */
   protected function doTestThemeCallbackAdministrative() {
     theme_enable(array($this->admin_theme));
     $this->drupalGet('menu-test/theme-callback/use-admin-theme');
-    $this->assertText('Custom theme: seven. Actual theme: seven.', 'The administrative theme can be correctly set in a theme callback.');
-    $this->assertRaw('seven/style.css', "The administrative theme's CSS appears on the page.");
-  }
-
-  /**
-   * Test that the theme callback is properly inherited.
-   */
-  protected function doTestThemeCallbackInheritance() {
-    theme_enable(array($this->admin_theme));
-    $this->drupalGet('menu-test/theme-callback/use-admin-theme/inheritance');
-    $this->assertText('Custom theme: seven. Actual theme: seven. Theme callback inheritance is being tested.', 'Theme callback inheritance correctly uses the administrative theme.');
+    $this->assertText('Active theme: seven. Actual theme: seven.', 'The administrative theme can be correctly set in a theme negotiation.');
     $this->assertRaw('seven/style.css', "The administrative theme's CSS appears on the page.");
   }
 
   /**
-   * Test the theme callback when the site is in maintenance mode.
+   * Test the theme negotiation when the site is in maintenance mode.
    */
   protected function doTestThemeCallbackMaintenanceMode() {
     $this->container->get('state')->set('system.maintenance_mode', TRUE);
@@ -551,76 +532,44 @@ protected function doTestThemeCallbackMaintenanceMode() {
     $admin_user = $this->drupalCreateUser(array('access site in maintenance mode'));
     $this->drupalLogin($admin_user);
     $this->drupalGet('menu-test/theme-callback/use-admin-theme');
-    $this->assertText('Custom theme: seven. Actual theme: seven.', 'The theme callback system is correctly triggered for an administrator when the site is in maintenance mode.');
+    $this->assertText('Active theme: seven. Actual theme: seven.', 'The theme negotiation system is correctly triggered for an administrator when the site is in maintenance mode.');
     $this->assertRaw('seven/style.css', "The administrative theme's CSS appears on the page.");
 
     $this->container->get('state')->set('system.maintenance_mode', FALSE);
   }
 
   /**
-   * Test the theme callback when it is set to use an optional theme.
+   * Test the theme negotiation when it is set to use an optional theme.
    */
   protected function doTestThemeCallbackOptionalTheme() {
     // Request a theme that is not enabled.
     $this->drupalGet('menu-test/theme-callback/use-stark-theme');
-    $this->assertText('Custom theme: NONE. Actual theme: bartik.', 'The theme callback system falls back on the default theme when a theme that is not enabled is requested.');
+    $this->assertText('Active theme: bartik. Actual theme: bartik.', 'The theme negotiation system falls back on the default theme when a theme that is not enabled is requested.');
     $this->assertRaw('bartik/css/style.css', "The default theme's CSS appears on the page.");
 
     // Now enable the theme and request it again.
     theme_enable(array($this->alternate_theme));
     $this->drupalGet('menu-test/theme-callback/use-stark-theme');
-    $this->assertText('Custom theme: stark. Actual theme: stark.', 'The theme callback system uses an optional theme once it has been enabled.');
+    $this->assertText('Active theme: stark. Actual theme: stark.', 'The theme negotiation system uses an optional theme once it has been enabled.');
     $this->assertRaw('stark/css/layout.css', "The optional theme's CSS appears on the page.");
   }
 
   /**
-   * Test the theme callback when it is set to use a theme that does not exist.
+   * Test the theme negotiation when it is set to use a theme that does not exist.
    */
   protected function doTestThemeCallbackFakeTheme() {
     $this->drupalGet('menu-test/theme-callback/use-fake-theme');
-    $this->assertText('Custom theme: NONE. Actual theme: bartik.', 'The theme callback system falls back on the default theme when a theme that does not exist is requested.');
+    $this->assertText('Active theme: bartik. Actual theme: bartik.', 'The theme negotiation system falls back on the default theme when a theme that does not exist is requested.');
     $this->assertRaw('bartik/css/style.css', "The default theme's CSS appears on the page.");
   }
 
   /**
-   * Test the theme callback when no theme is requested.
+   * Test the theme negotiation when no theme is requested.
    */
   protected function doTestThemeCallbackNoThemeRequested() {
     $this->drupalGet('menu-test/theme-callback/no-theme-requested');
-    $this->assertText('Custom theme: NONE. Actual theme: bartik.', 'The theme callback system falls back on the default theme when no theme is requested.');
+    $this->assertText('Active theme: bartik. Actual theme: bartik.', 'The theme negotiation system falls back on the default theme when no theme is requested.');
     $this->assertRaw('bartik/css/style.css', "The default theme's CSS appears on the page.");
   }
 
-  /**
-   * Test that hook_custom_theme() can control the theme of a page.
-   */
-  protected function doTestHookCustomTheme() {
-    // Trigger hook_custom_theme() to dynamically request the Stark theme for
-    // the requested page.
-    \Drupal::state()->set('menu_test.hook_custom_theme_name', $this->alternate_theme);
-    theme_enable(array($this->alternate_theme, $this->admin_theme));
-
-    // Visit a page that does not implement a theme callback. The above request
-    // should be honored.
-    $this->drupalGet('menu-test/no-theme-callback');
-    $this->assertText('Custom theme: stark. Actual theme: stark.', 'The result of hook_custom_theme() is used as the theme for the current page.');
-    $this->assertRaw('stark/css/layout.css', "The Stark theme's CSS appears on the page.");
-  }
-
-  /**
-   * Test that the theme callback wins out over hook_custom_theme().
-   */
-  protected function doTestThemeCallbackHookCustomTheme() {
-    // Trigger hook_custom_theme() to dynamically request the Stark theme for
-    // the requested page.
-    \Drupal::state()->set('menu_test.hook_custom_theme_name', $this->alternate_theme);
-    theme_enable(array($this->alternate_theme, $this->admin_theme));
-
-    // The menu "theme callback" should take precedence over a value set in
-    // hook_custom_theme().
-    $this->drupalGet('menu-test/theme-callback/use-admin-theme');
-    $this->assertText('Custom theme: seven. Actual theme: seven.', 'The result of hook_custom_theme() does not override what was set in a theme callback.');
-    $this->assertRaw('seven/style.css', "The Seven theme's CSS appears on the page.");
-  }
-
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php
index 9a13acbc0e4d8a1e0c0ebc40a36b29da76e3d5d9..554450accbc60ec837543b70b74e90294e675f4e 100644
--- a/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php
@@ -178,7 +178,7 @@ function testThemeSettings() {
    * Test the administration theme functionality.
    */
   function testAdministrationTheme() {
-    theme_enable(array('seven'));
+    theme_enable(array('bartik', 'seven'));
 
     // Enable an administration theme and show it on the node admin pages.
     $edit = array(
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
index 4a51db283db6646f77b54ebfd38fd43349ddf77c..492dfd666bf0fd83e7e1395af6f153abca639421 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -118,6 +118,17 @@ function testPreprocessForSuggestions() {
     }
   }
 
+  /**
+   * Tests the priority of some theme negotiators.
+   */
+  public function testNegotiatorPriorities() {
+    $this->drupalGet('theme-test/priority');
+
+    // Ensure that the custom theme negotiator was not able to set the theme.
+
+    $this->assertNoText('Theme hook implementor=test_theme_theme_test__suggestion(). Foo=template_preprocess_theme_test', 'Theme hook suggestion ran with data available from a preprocess function for the base hook.');
+  }
+
   /**
    * Ensure page-front template suggestion is added when on front page.
    */
@@ -268,5 +279,4 @@ function testPreprocessHtml() {
     $this->assertText('theme test page bottom markup', 'Modules are able to set the page bottom region.');
   }
 
-
 }
diff --git a/core/modules/system/lib/Drupal/system/Theme/BatchNegotiator.php b/core/modules/system/lib/Drupal/system/Theme/BatchNegotiator.php
new file mode 100644
index 0000000000000000000000000000000000000000..700250415a049ec568a14ba0338fffe62daa6983
--- /dev/null
+++ b/core/modules/system/lib/Drupal/system/Theme/BatchNegotiator.php
@@ -0,0 +1,59 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Theme\BatchNegotiator.
+ */
+
+namespace Drupal\system\Theme;
+
+use Drupal\Core\Batch\BatchStorageInterface;
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
+use Symfony\Cmf\Component\Routing\RouteObjectInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Sets the active theme for the batch page.
+ */
+class BatchNegotiator implements ThemeNegotiatorInterface {
+
+  /**
+   * The batch storage.
+   *
+   * @var \Drupal\Core\Batch\BatchStorageInterface
+   */
+  protected $batchStorage;
+
+  /**
+   * Constructs a BatchNegotiator.
+   *
+   * @param \Drupal\Core\Batch\BatchStorageInterface $batch_storage
+   *   The batch storage.
+   */
+  public function __construct(BatchStorageInterface $batch_storage) {
+    $this->batchStorage = $batch_storage;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(Request $request) {
+    return $request->attributes->get(RouteObjectInterface::ROUTE_NAME) == 'system.batch_page';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(Request $request) {
+    // Retrieve the current state of the batch.
+    $batch = &batch_get();
+    if (!$batch && $request->request->has('id')) {
+      $batch = $this->batchStorage->load($request->request->get('id'));
+    }
+    // Use the same theme as the page that started the batch.
+    if (!empty($batch['theme'])) {
+      return $batch['theme'];
+    }
+  }
+
+}
diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index 8513a28b50154f51035ca8c1558c7fae2a14ce71..cabea758d2d29e228986cdfad2b3edd97c233d87 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -1424,36 +1424,6 @@ function hook_template_preprocess_default_variables_alter(&$variables) {
   $variables['is_admin'] = user_access('access administration pages');
 }
 
-/**
- * Return the machine-readable name of the theme to use for the current page.
- *
- * This hook can be used to dynamically set the theme for the current page
- * request. It should be used by modules which need to override the theme
- * based on dynamic conditions (for example, a module which allows the theme to
- * be set based on the current user's role). The return value of this hook will
- * be used on all pages except those which have a valid per-page or per-section
- * theme set via a theme callback function in hook_menu(); the themes on those
- * pages can only be overridden using hook_menu_alter().
- *
- * Note that returning different themes for the same path may not work with page
- * caching. This is most likely to be a problem if an anonymous user on a given
- * path could have different themes returned under different conditions.
- *
- * Since only one theme can be used at a time, the last (i.e., highest
- * weighted) module which returns a valid theme name from this hook will
- * prevail.
- *
- * @return
- *   The machine-readable name of the theme that should be used for the current
- *   page request. The value returned from this function will only have an
- *   effect if it corresponds to a currently-active theme on the site. Do not
- *   return a value if you do not wish to set a custom theme.
- */
-function hook_custom_theme() {
-  // Allow the user to request a particular theme via a query parameter.
-  return \Drupal::request()->query->get('theme');
-}
-
 /**
  * Log an event message.
  *
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index 56b547e0f471e57a4a099e133a57a90a99e06555..14153a06599ec7b99755732e10fcb64fd6685d9d 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -833,20 +833,6 @@ function system_schema() {
         'not null' => TRUE,
         'default' => '',
       ),
-      'theme_callback' => array(
-        'description' => 'A function which returns the name of the theme that will be used to render this page. If left empty, the default theme will be used.',
-        'type' => 'varchar',
-        'length' => 255,
-        'not null' => TRUE,
-        'default' => '',
-      ),
-      'theme_arguments' => array(
-        'description' => 'A serialized array of arguments for the theme callback.',
-        'type' => 'varchar',
-        'length' => 255,
-        'not null' => TRUE,
-        'default' => '',
-      ),
       'type' => array(
         'description' => 'Numeric representation of the type of the menu item, like MENU_LOCAL_TASK.',
         'type' => 'int',
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index c725d8a7a0f05c04bb3300502a39a4f5f2ffb473..859b2a1931eba910bd791f319fb559879da1539b 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -609,12 +609,6 @@ function system_element_info() {
  * Implements hook_menu().
  */
 function system_menu() {
-  $items['system/ajax'] = array(
-    'title' => 'AHAH callback',
-    'route_name' => 'system.ajax',
-    'theme callback' => 'ajax_base_page_theme',
-    'type' => MENU_CALLBACK,
-  );
   $items['admin'] = array(
     'title' => 'Administration',
     'route_name' => 'system.admin',
@@ -808,13 +802,6 @@ function system_menu() {
     'route_name' => 'system.status',
   );
 
-  // Default page for batch operations.
-  $items['batch'] = array(
-    'route_name' => 'system.batch_page',
-    'theme callback' => '_system_batch_theme',
-    'type' => MENU_CALLBACK,
-  );
-
   return $items;
 }
 
@@ -866,21 +853,6 @@ function system_theme_suggestions_region(array $variables) {
   return $suggestions;
 }
 
-/**
- * Theme callback for the default batch page.
- */
-function _system_batch_theme() {
-  // Retrieve the current state of the batch.
-  $batch = &batch_get();
-  if (!$batch && isset($_REQUEST['id'])) {
-    $batch = \Drupal::service('batch.storage')->load($_REQUEST['id']);
-  }
-  // Use the same theme as the page that started the batch.
-  if (!empty($batch['theme'])) {
-    return $batch['theme'];
-  }
-}
-
 /**
  * Implements hook_library_info().
  */
@@ -2139,19 +2111,6 @@ function system_page_build(&$page) {
   }
 }
 
-/**
- * Implements hook_custom_theme().
- */
-function system_custom_theme() {
-  if (drupal_container()->isScopeActive('request')) {
-    $request = \Drupal::request();
-    $path = $request->attributes->get('_system_path');
-    if (user_access('view the administration theme') && path_is_admin($path)) {
-      return \Drupal::config('system.theme')->get('admin');
-    }
-  }
-}
-
 /**
  * Implements hook_form_FORM_ID_alter().
  */
diff --git a/core/modules/system/system.routing.yml b/core/modules/system/system.routing.yml
index c1a7a377f652f14f8f5800131495a0438e4290a0..25108cea94a4515a979fae3dfdd9a6e903be4573 100644
--- a/core/modules/system/system.routing.yml
+++ b/core/modules/system/system.routing.yml
@@ -2,6 +2,8 @@ system.ajax:
   path: '/system/ajax'
   defaults:
     _controller: '\Drupal\system\Controller\FormAjaxController::content'
+  options:
+    _theme: ajax_base_page
   requirements:
     _access: 'TRUE'
 
diff --git a/core/modules/system/system.services.yml b/core/modules/system/system.services.yml
index 935ddfd3e3f2d110ed67071b9c499db2db72c80b..3dcbb847e7b466145fa6ab169d4ee43afd2f2e63 100644
--- a/core/modules/system/system.services.yml
+++ b/core/modules/system/system.services.yml
@@ -15,3 +15,8 @@ services:
     class: Drupal\system\PathProcessor\PathProcessorFiles
     tags:
       - { name: path_processor_inbound, priority: 200 }
+  theme.negotiator.system.batch:
+    class: Drupal\system\Theme\BatchNegotiator
+    arguments: ['@batch.storage']
+    tags:
+      - { name: theme_negotiator, priority: 1000 }
diff --git a/core/modules/system/tests/modules/ajax_test/ajax_test.module b/core/modules/system/tests/modules/ajax_test/ajax_test.module
index f9974732b75ff7e77294d4e6dc8c9d761ba5857f..eba9efb532e3eb5868b30ecc4f039b39dba70e20 100644
--- a/core/modules/system/tests/modules/ajax_test/ajax_test.module
+++ b/core/modules/system/tests/modules/ajax_test/ajax_test.module
@@ -12,19 +12,6 @@
 use Drupal\Core\Ajax\CloseDialogCommand;
 use Drupal\Core\Ajax\HtmlCommand;
 
-/**
- * Implements hook_menu().
- */
-function ajax_test_menu() {
-  $items['ajax-test/order'] = array(
-    'title' => 'AJAX commands order',
-    'route_name' => 'ajax_test.order',
-    'theme callback' => 'ajax_base_page_theme',
-    'type' => MENU_CALLBACK,
-  );
-  return $items;
-}
-
 /**
  * Implements hook_system_theme_info().
  */
diff --git a/core/modules/system/tests/modules/ajax_test/ajax_test.routing.yml b/core/modules/system/tests/modules/ajax_test/ajax_test.routing.yml
index 34ce950d140c088a3f932cef118b72867d7d6359..2f92467c70c6ae12cb2a831392372aebaa6965d0 100644
--- a/core/modules/system/tests/modules/ajax_test/ajax_test.routing.yml
+++ b/core/modules/system/tests/modules/ajax_test/ajax_test.routing.yml
@@ -39,6 +39,8 @@ ajax_test.order:
   path: '/ajax-test/order'
   defaults:
     _controller: '\Drupal\ajax_test\Controller\AjaxTestController::order'
+  options:
+    _theme: ajax_base_page
   requirements:
     _access: 'TRUE'
 
diff --git a/core/modules/system/tests/modules/menu_test/lib/Drupal/menu_test/EventSubscriber/ActiveTrailSubscriber.php b/core/modules/system/tests/modules/menu_test/lib/Drupal/menu_test/EventSubscriber/ActiveTrailSubscriber.php
new file mode 100644
index 0000000000000000000000000000000000000000..04c79c12bbdc16f0b6b5c49c612de944456e4309
--- /dev/null
+++ b/core/modules/system/tests/modules/menu_test/lib/Drupal/menu_test/EventSubscriber/ActiveTrailSubscriber.php
@@ -0,0 +1,69 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\menu_test\EventSubscriber\ActiveTrailSubscriber.
+ */
+
+namespace Drupal\menu_test\EventSubscriber;
+
+use Drupal\Core\KeyValueStore\KeyValueStoreInterface;
+use Drupal\Core\KeyValueStore\StateInterface;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Symfony\Component\HttpKernel\Event\GetResponseEvent;
+use Symfony\Component\HttpKernel\KernelEvents;
+
+/**
+ * Tracks the active trail.
+ */
+class ActiveTrailSubscriber implements EventSubscriberInterface {
+
+  /**
+   * The active trail before redirect.
+   *
+   * @var array
+   */
+  protected $trail = array();
+
+  /**
+   * The state service.
+   *
+   * @var \Drupal\Core\KeyValueStore\StateInterface
+   */
+  protected $state;
+
+  /**
+   * Constructs a new ActiveTrailSubscriber.
+   *
+   * @param \Drupal\Core\KeyValueStore\StateInterface $state
+   *   The state service.
+   */
+  public function __construct(StateInterface $state) {
+    $this->state = $state;
+  }
+
+  /**
+   * Tracks the active trail.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
+   *   The event to process.
+   */
+  public function onKernelRequest(GetResponseEvent $event) {
+    // When requested by one of the MenuTrailTestCase tests, record the initial
+    // active trail during Drupal's bootstrap (before the user is redirected to
+    // a custom 403 or 404 page).
+    if (!$this->trail && $this->state->get('menu_test.record_active_trail') ?: FALSE) {
+      $this->trail = menu_get_active_trail();
+      $this->state->set('menu_test.active_trail_initial', $this->trail);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    $events[KernelEvents::REQUEST][] = array('onKernelRequest');
+    return $events;
+  }
+
+}
diff --git a/core/modules/system/tests/modules/menu_test/lib/Drupal/menu_test/Theme/TestThemeNegotiator.php b/core/modules/system/tests/modules/menu_test/lib/Drupal/menu_test/Theme/TestThemeNegotiator.php
new file mode 100644
index 0000000000000000000000000000000000000000..77954a35bd91a891ce1a9e9df879afb92b1a9141
--- /dev/null
+++ b/core/modules/system/tests/modules/menu_test/lib/Drupal/menu_test/Theme/TestThemeNegotiator.php
@@ -0,0 +1,50 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\menu_test\Theme\TestThemeNegotiator.
+ */
+
+namespace Drupal\menu_test\Theme;
+
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Tests the theme negotiation functionality.
+ *
+ * Retrieves the theme key of the theme to use for the current request based on
+ * the theme name provided in the URL.
+ */
+class TestThemeNegotiator implements ThemeNegotiatorInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(Request $request) {
+    return $request->attributes->has('inherited');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(Request $request) {
+    $argument = $request->attributes->get('inherited');
+    // Test using the variable administrative theme.
+    if ($argument == 'use-admin-theme') {
+      return \Drupal::config('system.theme')->get('admin');
+    }
+    // Test using a theme that exists, but may or may not be enabled.
+    elseif ($argument == 'use-stark-theme') {
+      return 'stark';
+    }
+    // Test using a theme that does not exist.
+    elseif ($argument == 'use-fake-theme') {
+      return 'fake_theme';
+    }
+    // For any other value of the URL argument, do not return anything. This
+    // allows us to test that returning nothing from a theme negotiation
+    // causes the page to correctly fall back on using the main site theme.
+  }
+
+}
diff --git a/core/modules/system/tests/modules/menu_test/menu_test.module b/core/modules/system/tests/modules/menu_test/menu_test.module
index 518cacefe2b5c1493e347f3e58d0761c6e8b2768..caf216bc63790560463321d80cebf601ac7221b8 100644
--- a/core/modules/system/tests/modules/menu_test/menu_test.module
+++ b/core/modules/system/tests/modules/menu_test/menu_test.module
@@ -64,18 +64,12 @@ function menu_test_menu() {
     'route_name' => 'menu_test.hierarchy_parent_child2',
   );
   // Theme callback tests.
-  $items['menu-test/theme-callback/%'] = array(
-    'title' => 'Page that displays different themes',
-    'route_name' => 'menu_test.theme_callback',
-    'theme callback' => 'menu_test_theme_callback',
-    'theme arguments' => array(2),
-  );
   $items['menu-test/theme-callback/%/inheritance'] = array(
-    'title' => 'Page that tests theme callback inheritance.',
+    'title' => 'Page that tests theme negotiation inheritance.',
     'route_name' => 'menu_test.theme_callback_inheritance',
   );
   $items['menu-test/no-theme-callback'] = array(
-    'title' => 'Page that displays different themes without using a theme callback.',
+    'title' => 'Page that displays different themes without using a theme negotiation.',
     'route_name' => 'menu_test.no_theme_callback',
   );
   // Path containing "exotic" characters.
@@ -444,7 +438,7 @@ function menu_test_custom_403_404_callback() {
 }
 
 /**
- * Page callback: Tests the theme callback functionality.
+ * Page callback: Tests the theme negotiation functionality.
  *
  * @param bool $inherited
  *   (optional) TRUE when the requested page is intended to inherit
@@ -462,66 +456,15 @@ function menu_test_theme_page_callback($inherited = FALSE) {
   global $theme_key;
   // Initialize the theme system so that $theme_key will be populated.
   drupal_theme_initialize();
-  // Now check both the requested custom theme and the actual theme being used.
-  $custom_theme = menu_get_custom_theme();
-  $requested_theme = empty($custom_theme) ? 'NONE' : $custom_theme;
-  $output = "Custom theme: $requested_theme. Actual theme: $theme_key.";
+  // Now we check what the theme negotiator service returns.
+  $active_theme = \Drupal::service('theme.negotiator')->getActiveTheme('getActiveTheme');
+  $output = "Active theme: $active_theme. Actual theme: $theme_key.";
   if ($inherited) {
-    $output .= ' Theme callback inheritance is being tested.';
+    $output .= ' Theme negotiation inheritance is being tested.';
   }
   return $output;
 }
 
-/**
- * Theme callback: Tests the theme callback functionality.
- *
- * Retrieves the theme key of the theme to use for the current request based on
- * the theme name provided in the URL.
- *
- * @param string $argument
- *   The argument passed in from the URL.
- *
- * @return string
- *   The name of the custom theme to request for the current page.
- *
- * @see menu_test_menu().
- */
-function menu_test_theme_callback($argument) {
-  // Test using the variable administrative theme.
-  if ($argument == 'use-admin-theme') {
-    return \Drupal::config('system.theme')->get('admin');
-  }
-  // Test using a theme that exists, but may or may not be enabled.
-  elseif ($argument == 'use-stark-theme') {
-    return 'stark';
-  }
-  // Test using a theme that does not exist.
-  elseif ($argument == 'use-fake-theme') {
-    return 'fake_theme';
-  }
-  // For any other value of the URL argument, do not return anything. This
-  // allows us to test that returning nothing from a theme callback function
-  // causes the page to correctly fall back on using the main site theme.
-}
-
-/**
- * Implements hook_custom_theme().
- *
- * If an appropriate variable has been set in the database, request the theme
- * that is stored there. Otherwise, do not attempt to dynamically set the theme.
- */
-function menu_test_custom_theme() {
-  // When requested by one of the MenuTrailTestCase tests, record the initial
-  // active trail during Drupal's bootstrap (before the user is redirected to a
-  // custom 403 or 404 page). See menu_test_custom_403_404_callback().
-  if (\Drupal::state()->get('menu_test.record_active_trail') ?: FALSE) {
-    \Drupal::state()->set('menu_test.active_trail_initial', menu_get_active_trail());
-  }
-  if ($theme = \Drupal::state()->get('menu_test.hook_custom_theme_name') ?: FALSE) {
-    return $theme;
-  }
-}
-
 /**
  * Sets a static variable for the testMenuName() test.
  *
diff --git a/core/modules/system/tests/modules/menu_test/menu_test.routing.yml b/core/modules/system/tests/modules/menu_test/menu_test.routing.yml
index cc333da0cfd30da29c886a61a7efcfe6809c7c1f..bc8a73682c8c7f0d8fe4e5507030b7d4ac940f1b 100644
--- a/core/modules/system/tests/modules/menu_test/menu_test.routing.yml
+++ b/core/modules/system/tests/modules/menu_test/menu_test.routing.yml
@@ -490,7 +490,7 @@ menu_test.theme_callback:
 menu_test.no_theme_callback:
   path: '/menu-test/no-theme-callback'
   defaults:
-    _title: 'Page that displays different themes without using a theme callback.'
+    _title: 'Page that displays different themes without using a theme negotiation.'
     _content: '\Drupal\menu_test\Controller\MenuTestController::themePage'
     inherited: false
   requirements:
@@ -511,7 +511,7 @@ menu_test.exotic_path:
 menu_test.theme_callback_inheritance:
   path: '/menu-test/theme-callback/{inherited}/inheritance'
   defaults:
-    _title: 'Page that tests theme callback inheritance.'
+    _title: 'Page that tests theme negotiation inheritance.'
     _content: '\Drupal\menu_test\Controller\MenuTestController::themePage'
   requirements:
     _permission: 'access content'
diff --git a/core/modules/system/tests/modules/menu_test/menu_test.services.yml b/core/modules/system/tests/modules/menu_test/menu_test.services.yml
index 097ddf201b9275a84ca6545d648f701d3730cae9..3de0169ad1071675d4c9ff2675a8f3477a353113 100644
--- a/core/modules/system/tests/modules/menu_test/menu_test.services.yml
+++ b/core/modules/system/tests/modules/menu_test/menu_test.services.yml
@@ -3,3 +3,14 @@ services:
     class: Drupal\menu_test\EventSubscriber\MaintenanceModeSubscriber
     tags:
       - { name: event_subscriber }
+
+  menu_test.active_trail_subscriber:
+    class: Drupal\menu_test\EventSubscriber\ActiveTrailSubscriber
+    arguments: ['@state']
+    tags:
+      - { name: event_subscriber }
+
+  theme.negotiator.test_theme:
+    class: Drupal\menu_test\Theme\TestThemeNegotiator
+    tags:
+      - { name: theme_negotiator }
diff --git a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/Theme/CustomThemeNegotiator.php b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/Theme/CustomThemeNegotiator.php
new file mode 100644
index 0000000000000000000000000000000000000000..33b9bec067a4cab7358ee494ce583c866f5a8da6
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/Theme/CustomThemeNegotiator.php
@@ -0,0 +1,35 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\theme_test\Theme\CustomThemeNegotiator.
+ */
+
+namespace Drupal\theme_test\Theme;
+
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
+use Symfony\Cmf\Component\Routing\RouteObjectInterface;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\Routing\Route;
+
+/**
+ * Just forces the 'test_theme' theme.
+ */
+class CustomThemeNegotiator implements ThemeNegotiatorInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(Request $request) {
+    return (($route_object = $request->attributes->get(RouteObjectInterface::ROUTE_OBJECT)) && $route_object instanceof Route && $route_object->hasOption('_custom_theme'));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(Request $request) {
+    $route_object = $request->attributes->get(RouteObjectInterface::ROUTE_OBJECT);
+    return $route_object->getOption('_custom_theme');
+  }
+
+}
diff --git a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/Theme/HighPriorityThemeNegotiator.php b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/Theme/HighPriorityThemeNegotiator.php
new file mode 100644
index 0000000000000000000000000000000000000000..91fc9365a167ccbe0da23fff28fddc0c468ac1c5
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/Theme/HighPriorityThemeNegotiator.php
@@ -0,0 +1,35 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\theme_test\Theme\HighPriorityThemeNegotiator.
+ */
+
+namespace Drupal\theme_test\Theme;
+
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
+use Symfony\Cmf\Component\Routing\RouteObjectInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Implements a test theme negotiator which was configured with a high priority.
+ */
+class HighPriorityThemeNegotiator implements ThemeNegotiatorInterface {
+
+
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(Request $request) {
+    return (($route_name = $request->attributes->get(RouteObjectInterface::ROUTE_NAME)) && $route_name == 'theme_test.priority');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(Request $request) {
+    return 'stark';
+  }
+
+}
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 87e26b8922836ee8eb3d44e01e835903db091629..52bff218b40d34e6e2d9b72c4ae393f597ec06a0 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -80,13 +80,6 @@ function theme_test_menu() {
   return $items;
 }
 
-/**
- * Custom theme callback.
- */
-function _theme_custom_theme() {
-  return 'test_theme';
-}
-
 /**
  * Implements hook_preprocess_HOOK() for HTML document templates.
  */
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.routing.yml b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
index b4a7fd64fe98576a8308ac7eafc5f8cdd868eb13..9f08d5b0c56ae2ded56233dcf65b5eb8f13d57c5 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
+++ b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
@@ -1,5 +1,7 @@
 theme_test.function_template_override:
   path: '/theme-test/function-template-overridden'
+  options:
+    _custom_theme: 'test_theme'
   defaults:
     _content: '\Drupal\theme_test\ThemeTestController::functionTemplateOverridden'
   requirements:
@@ -21,6 +23,18 @@ theme_test.template_test:
 
 theme_test.suggestion:
   path: '/theme-test/suggestion'
+  options:
+    _custom_theme: 'test_theme'
+  defaults:
+    _content: '\Drupal\theme_test\ThemeTestController::testSuggestion'
+    _title: 'Suggestion'
+  requirements:
+    _access: 'TRUE'
+
+theme_test.priority:
+  path: '/theme-test/priority'
+  options:
+    _custom_theme: 'test_theme'
   defaults:
     _content: '\Drupal\theme_test\ThemeTestController::testSuggestion'
     _title: 'Suggestion'
@@ -29,6 +43,8 @@ theme_test.suggestion:
 
 theme_test.alter:
   path: '/theme-test/alter'
+  options:
+    _custom_theme: 'test_theme'
   defaults:
     _content: '\Drupal\theme_test\ThemeTestController::testAlter'
     _title: 'Suggestion'
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.services.yml b/core/modules/system/tests/modules/theme_test/theme_test.services.yml
index 8e442df969599c7cdc56bd5db1eb7c6de1c9f4dc..69fd3ca7005764b3b11c653eb88a2b145ec238c8 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.services.yml
+++ b/core/modules/system/tests/modules/theme_test/theme_test.services.yml
@@ -3,3 +3,13 @@ services:
     class: Drupal\theme_test\EventSubscriber\ThemeTestSubscriber
     tags:
       - { name: event_subscriber }
+
+  theme.negotiator.test_custom_theme:
+    class: Drupal\theme_test\Theme\CustomThemeNegotiator
+    tags:
+      - { name: theme_negotiator }
+
+  theme.negotiator.high_priority:
+    class: Drupal\theme_test\Theme\HighPriorityThemeNegotiator
+    tags:
+      - { name: theme_negotiator, priority: 1000 }
diff --git a/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php b/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
index 452e33ff6242c32d81e24614077816429967ba0d..ee8cc38b3f38a91e144ac88ede73d32f7efcdaa1 100644
--- a/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
@@ -25,11 +25,11 @@ function setUp() {
 
     // Make sure we are using distinct default and administrative themes for
     // the duration of these tests.
+    theme_enable(array('bartik', 'seven'));
     \Drupal::config('system.theme')
       ->set('default', 'bartik')
+      ->set('admin', 'seven')
       ->save();
-    theme_enable(array('seven'));
-    \Drupal::config('system.theme')->set('admin', 'seven')->save();
 
     // Create and log in as a user who has permission to add and edit taxonomy
     // terms and view the administrative theme.
diff --git a/core/modules/text/lib/Drupal/text/Tests/Formatter/TextPlainUnitTest.php b/core/modules/text/lib/Drupal/text/Tests/Formatter/TextPlainUnitTest.php
index 9f986abbc189a032fa76abd37e44f4a039661cc7..38d184e4c05c8c924a050305758e0c0c0d3e3a0a 100644
--- a/core/modules/text/lib/Drupal/text/Tests/Formatter/TextPlainUnitTest.php
+++ b/core/modules/text/lib/Drupal/text/Tests/Formatter/TextPlainUnitTest.php
@@ -49,6 +49,7 @@ function setUp() {
     // Configure the theme system.
     $this->installConfig(array('system', 'field'));
     $this->installSchema('entity_test', 'entity_test');
+    $this->installSchema('user', 'users');
 
     // @todo Add helper methods for all of the following.
 
diff --git a/core/modules/user/lib/Drupal/user/Theme/AdminNegotiator.php b/core/modules/user/lib/Drupal/user/Theme/AdminNegotiator.php
new file mode 100644
index 0000000000000000000000000000000000000000..a12e1571525ac565a1e049ebfbd9643b8d1ca411
--- /dev/null
+++ b/core/modules/user/lib/Drupal/user/Theme/AdminNegotiator.php
@@ -0,0 +1,73 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\user\Theme\AdminNegotiator.
+ */
+
+namespace Drupal\user\Theme;
+
+use Drupal\Core\Config\ConfigFactory;
+use Drupal\Core\Entity\EntityManager;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Sets the active theme on admin pages.
+ */
+class AdminNegotiator implements ThemeNegotiatorInterface {
+
+  /**
+   * The current user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $user;
+
+  /**
+   * The config factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactory
+   */
+  protected $configFactory;
+
+  /**
+   * The entity manager.
+   *
+   * @var \Drupal\Core\Entity\EntityManager
+   */
+  protected $entityManager;
+
+  /**
+   * Creates a new AdminNegotiator instance.
+   *
+   * @param \Drupal\Core\Session\AccountInterface $user
+   *   The current user.
+   * @param \Drupal\Core\Config\ConfigFactory $config_factory
+   *   The config factory.
+   * @param \Drupal\Core\Entity\EntityManager $entity_manager
+   *   The entity manager.
+   */
+  public function __construct(AccountInterface $user, ConfigFactory $config_factory, EntityManager $entity_manager) {
+    $this->user = $user;
+    $this->configFactory = $config_factory;
+    $this->entityManager = $entity_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(Request $request) {
+    $path = $request->attributes->get('_system_path');
+    return ($this->entityManager->hasController('user_role', 'storage') && $this->user->hasPermission('view the administration theme') && path_is_admin($path));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(Request $request) {
+    return $this->configFactory->get('system.theme')->get('admin');
+  }
+
+}
diff --git a/core/modules/user/lib/Drupal/user/Theme/UserNegotiator.php b/core/modules/user/lib/Drupal/user/Theme/UserNegotiator.php
new file mode 100644
index 0000000000000000000000000000000000000000..f601acf6ca1f8edeff19b75d068265c730f80733
--- /dev/null
+++ b/core/modules/user/lib/Drupal/user/Theme/UserNegotiator.php
@@ -0,0 +1,68 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\user\Theme\UserNegotiator.
+ */
+
+namespace Drupal\user\Theme;
+
+use Drupal\Core\Entity\EntityManager;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\Theme\ActiveTheme;
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Defines the theme negotiator service for theme configured per user.
+ */
+class UserNegotiator implements  ThemeNegotiatorInterface {
+
+  /**
+   * The user storage controller.
+   *
+   * @var \Drupal\user\UserStorageControllerInterface
+   */
+  protected $userStorageController;
+
+  /**
+   * The current user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $currentUser;
+
+  /**
+   * Constructs a UserNegotiator object.
+   *
+   * @param \Drupal\Core\Entity\EntityManager $entity_manager
+   *   The entity manager
+   * @param \Drupal\Core\Session\AccountInterface $current_user
+   *   The current user.
+   */
+  public function __construct(EntityManager $entity_manager, AccountInterface $current_user) {
+    $this->userStorageController = $entity_manager->getStorageController('user');
+    $this->currentUser = $current_user;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(Request $request) {
+    return TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(Request $request) {
+    if ($user = $this->userStorageController->load($this->currentUser->id())) {;
+      // Only select the user selected theme if it is available in the
+      // list of themes that can be accessed.
+      if (!empty($user->theme) && drupal_theme_access($user->theme)) {
+        return $user->theme;
+      }
+    }
+  }
+
+}
diff --git a/core/modules/user/user.services.yml b/core/modules/user/user.services.yml
index 6fb7d47eeabff9fe3c4f9858ab936f107490b8e2..f35e710d4bf47d7c43de4fbf1968d818c4d50cb0 100644
--- a/core/modules/user/user.services.yml
+++ b/core/modules/user/user.services.yml
@@ -25,3 +25,13 @@ services:
     class: Drupal\user\EventSubscriber\MaintenanceModeSubscriber
     tags:
       - { name: event_subscriber }
+  theme.negotiator.user:
+    class: Drupal\user\Theme\UserNegotiator
+    arguments: ['@entity.manager', '@current_user']
+    tags:
+      - { name: theme_negotiator, priority: -50 }
+  theme.negotiator.admin_theme:
+    class: Drupal\user\Theme\AdminNegotiator
+    arguments: ['@current_user', '@config.factory', '@entity.manager']
+    tags:
+      - { name: theme_negotiator, priority: -40 }
diff --git a/core/modules/views/lib/Drupal/views/Tests/Handler/FieldCounterTest.php b/core/modules/views/lib/Drupal/views/Tests/Handler/FieldCounterTest.php
index 87732485d4588b5eb293fdda1902cac2cf1184e4..897bfd923de07ad43cad144d215b13183a0fa770 100644
--- a/core/modules/views/lib/Drupal/views/Tests/Handler/FieldCounterTest.php
+++ b/core/modules/views/lib/Drupal/views/Tests/Handler/FieldCounterTest.php
@@ -19,7 +19,7 @@ class FieldCounterTest extends ViewUnitTestBase {
    *
    * @var array
    */
-  public static $modules = array('user');
+  public static $modules = array('user', 'field');
 
   /**
    * Views used by this test.
@@ -36,6 +36,12 @@ public static function getInfo() {
     );
   }
 
+  protected function setUp() {
+    parent::setUp();
+
+    $this->installSchema('user', 'users');
+  }
+
   function testSimple() {
     $view = views_get_view('test_view');
     $view->setDisplay();
diff --git a/core/modules/views/lib/Drupal/views/Tests/Handler/FieldUnitTest.php b/core/modules/views/lib/Drupal/views/Tests/Handler/FieldUnitTest.php
index 6ad3b19a4d79cf68c72e147c1a086ed479063238..affd9bdc526ee302d51ec154c56f1b591b428e03 100644
--- a/core/modules/views/lib/Drupal/views/Tests/Handler/FieldUnitTest.php
+++ b/core/modules/views/lib/Drupal/views/Tests/Handler/FieldUnitTest.php
@@ -17,7 +17,7 @@
  */
 class FieldUnitTest extends ViewUnitTestBase {
 
-  public static $modules = array('user');
+  public static $modules = array('user', 'field');
 
   /**
    * Views used by this test.
@@ -38,6 +38,12 @@ public static function getInfo() {
     );
   }
 
+  protected function setUp() {
+    parent::setUp();
+
+    $this->installSchema('user', 'users');
+  }
+
   /**
    * Overrides Drupal\views\Tests\ViewTestBase::viewsData().
    */
diff --git a/core/modules/views/lib/Drupal/views/Tests/Plugin/DisplayPageTest.php b/core/modules/views/lib/Drupal/views/Tests/Plugin/DisplayPageTest.php
index 48c116e201dc9e4ffd7a5b4b310d3fa3ff73505e..b4e343d407208b1f1c066f9ca94153d03765205f 100644
--- a/core/modules/views/lib/Drupal/views/Tests/Plugin/DisplayPageTest.php
+++ b/core/modules/views/lib/Drupal/views/Tests/Plugin/DisplayPageTest.php
@@ -59,6 +59,7 @@ protected function setUp() {
     // Setup the needed tables in order to make the drupal router working.
     $this->installSchema('system', array('router', 'menu_router', 'url_alias'));
     $this->installSchema('menu_link', 'menu_links');
+    $this->installSchema('user', 'users');
   }
 
   /**
diff --git a/core/modules/views/lib/Drupal/views/ViewExecutable.php b/core/modules/views/lib/Drupal/views/ViewExecutable.php
index 60884d6bfc9e8fcf4f77ed20706d515aac7b4c0b..f6a0129cd790ec282dababe98aab5afaafc7788a 100644
--- a/core/modules/views/lib/Drupal/views/ViewExecutable.php
+++ b/core/modules/views/lib/Drupal/views/ViewExecutable.php
@@ -1278,8 +1278,6 @@ public function render($display_id = NULL) {
       return;
     }
 
-    drupal_theme_initialize();
-
     $exposed_form = $this->display_handler->getPlugin('exposed_form');
     $exposed_form->preRender($this->result);
 
@@ -1341,11 +1339,13 @@ public function render($display_id = NULL) {
       $module_handler->invokeAll('views_pre_render', array($this));
 
       // Let the themes play too, because pre render is a very themey thing.
-      foreach ($GLOBALS['base_theme_info'] as $base) {
-        $module_handler->invoke($base, 'views_pre_render', array($this));
-      }
+      if (isset($GLOBALS['base_theme_info']) && isset($GLOBALS['theme'])) {
+        foreach ($GLOBALS['base_theme_info'] as $base) {
+          $module_handler->invoke($base, 'views_pre_render', array($this));
+        }
 
-      $module_handler->invoke($GLOBALS['theme'], 'views_pre_render', array($this));
+        $module_handler->invoke($GLOBALS['theme'], 'views_pre_render', array($this));
+      }
 
       $this->display_handler->output = $this->display_handler->render();
       if ($cache) {
@@ -1363,11 +1363,13 @@ public function render($display_id = NULL) {
     $module_handler->invokeAll('views_post_render', array($this, &$this->display_handler->output, $cache));
 
     // Let the themes play too, because post render is a very themey thing.
-    foreach ($GLOBALS['base_theme_info'] as $base) {
-      $module_handler->invoke($base, 'views_post_render', array($this));
-    }
+    if (isset($GLOBALS['base_theme_info']) && isset($GLOBALS['theme'])) {
+      foreach ($GLOBALS['base_theme_info'] as $base) {
+        $module_handler->invoke($base, 'views_post_render', array($this));
+      }
 
-    $module_handler->invoke($GLOBALS['theme'], 'views_post_render', array($this));
+      $module_handler->invoke($GLOBALS['theme'], 'views_post_render', array($this));
+    }
 
     return $this->display_handler->output;
   }
diff --git a/core/modules/views/views.module b/core/modules/views/views.module
index dda4f403ba508e37f3d4d196d4b4850d46df8ef0..4ac7613886d0161528bce51fc8716f980afc200f 100644
--- a/core/modules/views/views.module
+++ b/core/modules/views/views.module
@@ -269,20 +269,6 @@ function views_permission() {
   );
 }
 
-/**
- * Implement hook_menu().
- */
-function views_menu() {
-  $items = array();
-  $items['views/ajax'] = array(
-    'title' => 'Views',
-    'theme callback' => 'ajax_base_page_theme',
-    'route_name' => 'views.ajax',
-    'type' => MENU_CALLBACK,
-  );
-  return $items;
-}
-
 /**
  * Implement hook_menu_alter().
  */
diff --git a/core/modules/views/views.routing.yml b/core/modules/views/views.routing.yml
index 9abe5d59142a7dfc22bc93e83289765f9ed94583..691640d00ea0a7076cd76902288d3e948d97a0de 100644
--- a/core/modules/views/views.routing.yml
+++ b/core/modules/views/views.routing.yml
@@ -2,5 +2,7 @@ views.ajax:
   path: '/views/ajax'
   defaults:
     _controller: '\Drupal\views\Controller\ViewAjaxController::ajaxView'
+  options:
+    _theme: ajax_base_page
   requirements:
     _access: 'TRUE'
diff --git a/core/tests/Drupal/Tests/Core/Theme/ThemeNegotiatorTest.php b/core/tests/Drupal/Tests/Core/Theme/ThemeNegotiatorTest.php
new file mode 100644
index 0000000000000000000000000000000000000000..e8bb2fe0be27fd200c79890f33d5de8398cda732
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Theme/ThemeNegotiatorTest.php
@@ -0,0 +1,193 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Theme\ThemeNegotiatorTest.
+ */
+
+namespace Drupal\Tests\Core\Theme;
+
+use Drupal\Core\Theme\ThemeNegotiator;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Tests the theme negotiator.
+ *
+ * @group Drupal
+ *
+ * @see \Drupal\Core\Theme\ThemeNegotiator
+ */
+class ThemeNegotiatorTest extends UnitTestCase {
+
+  /**
+   * The mocked theme access checker.
+   *
+   * @var \Drupal\Core\Theme\ThemeAccessCheck|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $themeAccessCheck;
+
+  /**
+   * The actual tested theme negotiator.
+   *
+   * @var \Drupal\Core\Theme\ThemeNegotiator
+   */
+  protected $themeNegotiator;
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Theme negotiator',
+      'description' => 'Tests the theme negotiator.',
+      'group' => 'Theme',
+    );
+  }
+
+  protected function setUp() {
+    $this->themeAccessCheck = $this->getMockBuilder('\Drupal\Core\Theme\ThemeAccessCheck')
+      ->disableOriginalConstructor()
+      ->getMock();
+    $this->themeNegotiator = new ThemeNegotiator($this->themeAccessCheck);
+  }
+
+  /**
+   * Tests determining the theme.
+   *
+   * @see \Drupal\Core\Theme\ThemeNegotiator::determineActiveTheme()
+   */
+  public function testDetermineActiveTheme() {
+    $negotiator = $this->getMock('Drupal\Core\Theme\ThemeNegotiatorInterface');
+    $negotiator->expects($this->once())
+      ->method('determineActiveTheme')
+      ->will($this->returnValue('example_test'));
+    $negotiator->expects($this->once())
+      ->method('applies')
+      ->will($this->returnValue(TRUE));
+
+    $this->themeNegotiator->addNegotiator($negotiator, 0);
+
+    $this->themeAccessCheck->expects($this->any())
+      ->method('checkAccess')
+      ->will($this->returnValue(TRUE));
+
+    $request = Request::create('/test-route');
+    $theme = $this->themeNegotiator->determineActiveTheme($request);
+
+    $this->assertEquals('example_test', $theme);
+    $this->assertEquals('example_test', $request->attributes->get('_theme_active'));
+  }
+
+  /**
+   * Tests determining with two negotiators checking the priority.
+   *
+   * @see \Drupal\Core\Theme\ThemeNegotiator::determineActiveTheme()
+   */
+  public function testDetermineActiveThemeWithPriority() {
+    $negotiator = $this->getMock('Drupal\Core\Theme\ThemeNegotiatorInterface');
+    $negotiator->expects($this->once())
+      ->method('determineActiveTheme')
+      ->will($this->returnValue('example_test'));
+    $negotiator->expects($this->once())
+      ->method('applies')
+      ->will($this->returnValue(TRUE));
+
+    $this->themeNegotiator->addNegotiator($negotiator, 10);
+
+    $negotiator = $this->getMock('Drupal\Core\Theme\ThemeNegotiatorInterface');
+    $negotiator->expects($this->never())
+      ->method('determineActiveTheme');
+    $negotiator->expects($this->never())
+      ->method('applies');
+
+    $this->themeNegotiator->addNegotiator($negotiator, 0);
+
+    $this->themeAccessCheck->expects($this->any())
+      ->method('checkAccess')
+      ->will($this->returnValue(TRUE));
+
+    $request = Request::create('/test-route');
+    $theme = $this->themeNegotiator->determineActiveTheme($request);
+
+    $this->assertEquals('example_test', $theme);
+    $this->assertEquals('example_test', $request->attributes->get('_theme_active'));
+  }
+
+  /**
+   * Tests determining with two negotiators of which just one returns access.
+   *
+   * @see \Drupal\Core\Theme\ThemeNegotiator::determineActiveTheme()
+   */
+  public function testDetermineActiveThemeWithAccessCheck() {
+    $negotiator = $this->getMock('Drupal\Core\Theme\ThemeNegotiatorInterface');
+    $negotiator->expects($this->once())
+      ->method('determineActiveTheme')
+      ->will($this->returnValue('example_test'));
+    $negotiator->expects($this->once())
+      ->method('applies')
+      ->will($this->returnValue(TRUE));
+
+    $this->themeNegotiator->addNegotiator($negotiator, 10);
+
+    $negotiator = $this->getMock('Drupal\Core\Theme\ThemeNegotiatorInterface');
+    $negotiator->expects($this->once())
+      ->method('determineActiveTheme')
+      ->will($this->returnValue('example_test2'));
+    $negotiator->expects($this->once())
+      ->method('applies')
+      ->will($this->returnValue(TRUE));
+
+    $this->themeNegotiator->addNegotiator($negotiator, 0);
+
+    $this->themeAccessCheck->expects($this->at(0))
+      ->method('checkAccess')
+      ->with('example_test')
+      ->will($this->returnValue(FALSE));
+
+    $this->themeAccessCheck->expects($this->at(1))
+      ->method('checkAccess')
+      ->with('example_test2')
+      ->will($this->returnValue(TRUE));
+
+    $request = Request::create('/test-route');
+    $theme = $this->themeNegotiator->determineActiveTheme($request);
+
+    $this->assertEquals('example_test2', $theme);
+    $this->assertEquals('example_test2', $request->attributes->get('_theme_active'));
+  }
+
+  /**
+   * Tests determining with two negotiators of which one does not apply.
+   *
+   * @see \Drupal\Core\Theme\ThemeNegotiatorInterface
+   */
+  public function testDetermineActiveThemeWithNotApplyingNegotiator() {
+    $negotiator = $this->getMock('Drupal\Core\Theme\ThemeNegotiatorInterface');
+    $negotiator->expects($this->never())
+      ->method('determineActiveTheme');
+    $negotiator->expects($this->once())
+      ->method('applies')
+      ->will($this->returnValue(FALSE));
+
+    $this->themeNegotiator->addNegotiator($negotiator, 10);
+
+    $negotiator = $this->getMock('Drupal\Core\Theme\ThemeNegotiatorInterface');
+    $negotiator->expects($this->once())
+      ->method('determineActiveTheme')
+      ->will($this->returnValue('example_test2'));
+    $negotiator->expects($this->once())
+      ->method('applies')
+      ->will($this->returnValue(TRUE));
+
+    $this->themeNegotiator->addNegotiator($negotiator, 0);
+
+    $this->themeAccessCheck->expects($this->any())
+      ->method('checkAccess')
+      ->will($this->returnValue(TRUE));
+
+    $request = Request::create('/test-route');
+    $theme = $this->themeNegotiator->determineActiveTheme($request);
+
+    $this->assertEquals('example_test2', $theme);
+    $this->assertEquals('example_test2', $request->attributes->get('_theme_active'));
+  }
+
+}