Issue #3209048 by jedihe, jplana: Core themes are not added to the test autoloader

core/tests/Drupal/Tests/Core/Theme/CoreThemesAutoloadedForTests.php

+<?php
+
+namespace Drupal\Tests\Core\Theme;
+
+use Drupal\Tests\UnitTestCase;
+use Drupal\claro\ClaroPreRender;
+
+/**
+ * Confirms that core/themes is autoloaded for tests.
+ *
+ * @group Theme
+ */
+class CoreThemesAutoloadedForTests extends UnitTestCase {
+
+  /**
+   * Confirms that core/themes is autoloaded for tests.
+   */
+  public function testCoreThemesAutoloadedForTests() {
+    $this->assertTrue(class_exists(ClaroPreRender::class), 'core/themes (ClaroPreRender) is registered with the tests autoloader');
+  }
+
+}

core/tests/bootstrap.php

@@ -51,6 +51,7 @@ function drupal_phpunit_contrib_extension_directory_roots($root = NULL) {
   $paths = [
     $root . '/core/modules',
     $root . '/core/profiles',
+    $root . '/core/themes',
     $root . '/modules',
     $root . '/profiles',
     $root . '/themes',