Issue #1992606 by damiankloip, pwieck: Convert system_theme_settings() to FormInterface.

core/includes/theme.inc

core/lib/Drupal/Core/Controller/HtmlFormController.php

...es/block/lib/Drupal/block/Tests/BlockHiddenRegionTest.php

core/modules/color/lib/Drupal/color/Tests/ColorTest.php

...dules/system/lib/Drupal/system/Form/ThemeSettingsForm.php

...ules/system/lib/Drupal/system/Routing/RouteSubscriber.php

...es/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php

...dules/system/lib/Drupal/system/Tests/System/ThemeTest.php

core/modules/system/system.admin.inc

core/modules/system/system.module

core/modules/system/system.routing.yml

core/modules/system/system.services.yml