diff --git a/core/core.services.yml b/core/core.services.yml
index 8a24ee7..7e90470 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -152,6 +152,19 @@ services:
     calls:
       - [addSubscriber, ['@http_client_simpletest_subscriber']]
       - [setUserAgent, ['Drupal (+http://drupal.org/)']]
+  theme.negotiator:
+    class: Drupal\Core\Theme\ThemeNegotiator
+    arguments: ['@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']
+    tags:
+      - { name: theme_negotiator }
   container.namespaces:
     class: ArrayObject
     arguments: [ '%container.namespaces%' ]
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 122cc63..ebe406c 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -93,16 +93,24 @@ 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');
+  $themes = list_themes();
 
+  // @todo Let the theme.negotiator listen to the kernel request event.
+  $request = Drupal::request();
+  Drupal::service('theme.negotiator')->determineActiveTheme($request);
+
+  // @todo Convert to a theme negotiator.
+  //   service.
   // 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 : $theme;
+  if ($custom_theme) {
+    $request->attributes->set('_theme_active', $custom_theme);
+  }
+
+  $theme = $request->attributes->get('_theme_active');
 
   // Store the identifier for retrieving theme settings with.
   $theme_key = $theme;
@@ -117,7 +125,7 @@ function drupal_theme_initialize() {
   _drupal_theme_initialize($themes[$theme], array_reverse($base_theme));
 
   // Themes can have alter functions, so reset the drupal_alter() cache.
-  drupal_static_reset('drupal_alter');
+  Drupal::moduleHandler()->resetImplementations();
 }
 
 /**
diff --git a/core/lib/Drupal/Core/CoreServiceProvider.php b/core/lib/Drupal/Core/CoreServiceProvider.php
index a6cb957..c3c6df5 100644
--- a/core/lib/Drupal/Core/CoreServiceProvider.php
+++ b/core/lib/Drupal/Core/CoreServiceProvider.php
@@ -22,6 +22,7 @@
 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;
@@ -68,6 +69,9 @@ 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/Theme/AjaxBasePageNegotiator.php b/core/lib/Drupal/Core/Theme/AjaxBasePageNegotiator.php
new file mode 100644
index 0000000..2cecad9
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/AjaxBasePageNegotiator.php
@@ -0,0 +1,69 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Theme\AjaxBasePageNegotiator.
+ */
+
+namespace Drupal\Core\Theme;
+
+use Drupal\Core\Access\CsrfTokenGenerator;
+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 '_ajax_base_page_theme' as part of the router options.
+ */
+class AjaxBasePageNegotiator implements ThemeNegotiatorInterface {
+
+  /**
+   * The CSRF token generator.
+   *
+   * @var \Drupal\Core\Access\CsrfTokenGenerator
+   */
+  protected $csrfGenerator;
+
+  /**
+   * Constructs a new AjaxBasePageNegotiator.
+   *
+   * @param \Drupal\Core\Access\CsrfTokenGenerator $token_generator
+   *   The CSRF token generator.
+   */
+  public function __construct(CsrfTokenGenerator $token_generator) {
+    $this->csrfGenerator = $token_generator;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(Request $request) {
+    // Check whether the route was configured to use the base page theme.
+    if (!(($route = $request->attributes->get(RouteObjectInterface::ROUTE_OBJECT)) && $route->hasOption('_ajax_base_page_theme'))) {
+      return NULL;
+    }
+    if (($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 = $ajax_page_state['theme_token'];
+
+      // Ensure that the user only access a theme they are allowed to see.
+      if ($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 0000000..f155b0d
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/DefaultNegotiator.php
@@ -0,0 +1,35 @@
+<?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 {
+
+  /**
+   * 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 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 0000000..5db1885
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/ThemeNegotiator.php
@@ -0,0 +1,115 @@
+<?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;
+
+  /**
+   * 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 breadcrumb builder.
+   */
+  public function addNegotiator(ThemeNegotiatorInterface $negotiator, $priority) {
+    $this->negotiators[$priority][] = $negotiator;
+    // Force the negotiators to be re-sorted.
+    $this->sortedNegotiators = NULL;
+  }
+
+  /**
+   * Returns the sorted array of theme negotiators.
+   *
+   * @return array|\Drupal\Core\Theme\ThemeNegotiatorInterface[]
+   *   An array of breadcrumb builder objects.
+   */
+  protected function getSortedNegotiators() {
+    if (!isset($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 determineActiveTheme(Request $request) {
+    foreach ($this->getSortedNegotiators() as $negotiator) {
+      if (($active_theme = $negotiator->determineActiveTheme($request))) {
+        $request->attributes->set('_theme_active', $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 0000000..9bb80b1
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/ThemeNegotiatorInterface.php
@@ -0,0 +1,28 @@
+<?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.
+ */
+interface ThemeNegotiatorInterface {
+
+  /**
+   * 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 0000000..7f14692
--- /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 bdff14b..67a8a4e 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 = Drupal.settings.ajaxPageState;
diff --git a/core/modules/contextual/contextual.module b/core/modules/contextual/contextual.module
index 10d57b1..a8bee3a 100644
--- a/core/modules/contextual/contextual.module
+++ b/core/modules/contextual/contextual.module
@@ -6,20 +6,6 @@
  */
 
 /**
- * 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',
-  );
-
-  return $items;
-}
-
-/**
  * Implements hook_toolbar().
  */
 function contextual_toolbar() {
diff --git a/core/modules/contextual/contextual.routing.yml b/core/modules/contextual/contextual.routing.yml
index 5dd4457..0286cad 100644
--- a/core/modules/contextual/contextual.routing.yml
+++ b/core/modules/contextual/contextual.routing.yml
@@ -1,5 +1,7 @@
 contextual_render:
   pattern: '/contextual/render'
+  options:
+    _ajax_base_page_theme: 'TRUE'
   defaults:
     _controller: '\Drupal\contextual\ContextualController::render'
   requirements:
diff --git a/core/modules/edit/edit.module b/core/modules/edit/edit.module
index a09889c..f0802eb 100644
--- a/core/modules/edit/edit.module
+++ b/core/modules/edit/edit.module
@@ -18,24 +18,6 @@
 use Drupal\user\TempStoreFactory;
 
 /**
- * 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',
-  );
-  $items['edit/form/%/%/%/%/%'] = array(
-    'route_name' => 'edit_field_form',
-    'theme callback' => 'ajax_base_page_theme',
-  );
-
-  return $items;
-}
-
-/**
  * Implements hook_permission().
  */
 function edit_permission() {
diff --git a/core/modules/edit/edit.routing.yml b/core/modules/edit/edit.routing.yml
index b709d0a..d310d45 100644
--- a/core/modules/edit/edit.routing.yml
+++ b/core/modules/edit/edit.routing.yml
@@ -1,5 +1,7 @@
 edit_metadata:
   pattern: '/edit/metadata'
+  options:
+    _ajax_base_page_theme: 'TRUE'
   defaults:
     _controller: '\Drupal\edit\EditController::metadata'
   requirements:
@@ -14,6 +16,8 @@ edit_attachments:
 
 edit_field_form:
   pattern: '/edit/form/{entity_type}/{entity}/{field_name}/{langcode}/{view_mode_id}'
+  options:
+    _ajax_base_page_theme: 'TRUE'
   defaults:
     _controller: '\Drupal\edit\EditController::fieldForm'
   requirements:
diff --git a/core/modules/editor/editor.module b/core/modules/editor/editor.module
index e9a44f0..44526ea 100644
--- a/core/modules/editor/editor.module
+++ b/core/modules/editor/editor.module
@@ -137,20 +137,6 @@ function editor_library_info() {
 }
 
 /**
- * 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',
-  );
-
-  return $items;
-}
-
-/**
  * Implements hook_form_FORM_ID_alter().
  */
 function editor_form_filter_admin_overview_alter(&$form, $form_state) {
diff --git a/core/modules/editor/editor.routing.yml b/core/modules/editor/editor.routing.yml
index f905634..783ff30 100644
--- a/core/modules/editor/editor.routing.yml
+++ b/core/modules/editor/editor.routing.yml
@@ -4,6 +4,7 @@ editor_field_untransformed_text:
     _controller: '\Drupal\editor\EditorController::getUntransformedText'
   options:
     _access_mode: 'ALL'
+    _ajax_base_page_theme: 'TRUE'
   requirements:
     _permission: 'access in-place editing'
     _access_edit_entity_field: 'TRUE'
diff --git a/core/modules/file/file.module b/core/modules/file/file.module
index 3bf9f7b..99cff1a 100644
--- a/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -39,26 +39,6 @@ function file_help($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,
-  );
-  $items['file/progress'] = array(
-    'route name' => 'file.ajax.progress',
-    'theme callback' => 'ajax_base_page_theme',
-    'type' => MENU_CALLBACK,
-  );
-
-  return $items;
-}
-
-/**
  * Implements hook_element_info().
  *
  * The managed file element may be used anywhere in Drupal.
diff --git a/core/modules/file/file.routing.yml b/core/modules/file/file.routing.yml
index c2a9efd..bd566fb 100644
--- a/core/modules/file/file.routing.yml
+++ b/core/modules/file/file.routing.yml
@@ -1,11 +1,15 @@
 file.ajax.upload:
   pattern: '/file/ajax'
+  options:
+    _ajax_base_page_theme: 'TRUE'
   defaults:
     _controller: '\Drupal\file\Controller\FileWidgetAjaxController::upload'
   requirements:
     _permission: 'access content'
 file.ajax.progress:
   pattern: '/file/progress'
+  options:
+    _ajax_base_page_theme: 'TRUE'
   defaults:
     _controller: '\Drupal\file\Controller\FileWidgetAjaxController::progress'
   requirements:
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index bd58f93..791f22d 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -613,12 +613,6 @@ function system_menu() {
     'access callback' => TRUE,
     'type' => MENU_CALLBACK,
   );
-  $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',
     'access arguments' => array('access administration pages'),
diff --git a/core/modules/system/system.routing.yml b/core/modules/system/system.routing.yml
index f12ba8a..b9edac9 100644
--- a/core/modules/system/system.routing.yml
+++ b/core/modules/system/system.routing.yml
@@ -1,5 +1,7 @@
 system.ajax:
   pattern: '/system/ajax'
+  options:
+    _ajax_base_page_theme: 'TRUE'
   defaults:
     _controller: '\Drupal\system\Controller\FormAjaxController::content'
   requirements:
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 0000000..17c1e33
--- /dev/null
+++ b/core/modules/user/lib/Drupal/user/Theme/UserNegotiator.php
@@ -0,0 +1,61 @@
+<?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 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 $user->theme;
+      }
+    }
+  }
+
+}
diff --git a/core/modules/user/user.services.yml b/core/modules/user/user.services.yml
index 6fb7d47..c70cad1 100644
--- a/core/modules/user/user.services.yml
+++ b/core/modules/user/user.services.yml
@@ -25,3 +25,9 @@ services:
     class: Drupal\user\EventSubscriber\MaintenanceModeSubscriber
     tags:
       - { name: event_subscriber }
+  theme.negotiator.user:
+    class: Drupal\user\Theme\UserNegotiator
+    arguments: ['@plugin.manager.entity', '@current_user']
+    tags:
+      - { name: theme_negotiator, priority: 50 }
+
diff --git a/core/modules/views/lib/Drupal/views/ViewExecutable.php b/core/modules/views/lib/Drupal/views/ViewExecutable.php
index 887c600..48ba1e1 100644
--- a/core/modules/views/lib/Drupal/views/ViewExecutable.php
+++ b/core/modules/views/lib/Drupal/views/ViewExecutable.php
@@ -1277,7 +1277,6 @@ public function render($display_id = NULL) {
       return;
     }
 
-    drupal_theme_initialize();
     $config = \Drupal::config('views.settings');
 
     $exposed_form = $this->display_handler->getPlugin('exposed_form');
@@ -1341,11 +1340,13 @@ public function render($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 +1364,13 @@ 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 26a9dbb..6219a7e 100644
--- a/core/modules/views/views.module
+++ b/core/modules/views/views.module
@@ -316,12 +316,6 @@ function views_menu() {
   // Views data is updated -- module changes, profile changes, etc.
   views_invalidate_cache();
   $items = array();
-  $items['views/ajax'] = array(
-    'title' => 'Views',
-    'theme callback' => 'ajax_base_page_theme',
-    'route_name' => 'views_ajax',
-    'type' => MENU_CALLBACK,
-  );
   // Define another taxonomy autocomplete because the default one of drupal
   // does not support a vid a argument anymore
   $items['admin/views/ajax/autocomplete/taxonomy/%'] = array(
diff --git a/core/modules/views/views.routing.yml b/core/modules/views/views.routing.yml
index 016b2a5..0e331ca 100644
--- a/core/modules/views/views.routing.yml
+++ b/core/modules/views/views.routing.yml
@@ -1,5 +1,7 @@
 views_ajax:
   pattern: '/views/ajax'
+  options:
+    _ajax_base_page_theme: 'TRUE'
   defaults:
     _controller: '\Drupal\views\Controller\ViewAjaxController::ajaxView'
   requirements: