diff --git a/core/core.services.yml b/core/core.services.yml
index cb83270..f71a5ee 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -153,6 +153,19 @@ 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 094a339..893971a 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -92,16 +92,24 @@ 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 all theme callbacks to theme negotiators.
   // 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 = $theme;
@@ -116,7 +124,7 @@ function drupal_theme_initialize() {
   _drupal_theme_initialize($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 9cd8f5a..4bd88ee 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;
@@ -69,6 +70,9 @@ 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 39828ef..d7ee895 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
diff --git a/core/lib/Drupal/Core/Theme/AjaxBasePageNegotiator.php b/core/lib/Drupal/Core/Theme/AjaxBasePageNegotiator.php
new file mode 100644
index 0000000..8c412fe
--- /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 a586945..e9baf94 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 f1112d6..108f6ba 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -134,13 +134,6 @@ function block_menu() {
       'type' => $key == $default_theme ? MENU_DEFAULT_LOCAL_TASK : MENU_LOCAL_TASK,
       'route_name' => "block.admin_display_$key",
     );
-    $items["admin/structure/block/demo/$key"] = array(
-      'title' => check_plain($theme->info['name']),
-      'route_name' => 'block.admin_demo',
-      'type' => MENU_CALLBACK,
-      'theme callback' => '_block_custom_theme',
-      'theme arguments' => array($key),
-    );
   }
   return $items;
 }
@@ -162,25 +155,6 @@ function _block_themes_access($theme) {
 }
 
 /**
- * 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().
  *
  * Renders blocks into their regions.
diff --git a/core/modules/block/block.services.yml b/core/modules/block/block.services.yml
index c6704bc..39b414a 100644
--- a/core/modules/block/block.services.yml
+++ b/core/modules/block/block.services.yml
@@ -13,3 +13,7 @@ services:
     class: Drupal\block\Routing\RouteSubscriber
     tags:
       - { name: event_subscriber}
+  theme.negotiator.block.admin_demo:
+    class: Drupal\block\Theme\AdminDemoNegotiator
+    tags:
+      - { name: theme_negotiator }
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 0000000..2d9d351
--- /dev/null
+++ b/core/modules/block/lib/Drupal/block/Theme/AdminDemoNegotiator.php
@@ -0,0 +1,30 @@
+<?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 determineActiveTheme(Request $request) {
+    // We return exactly what was passed in, to guarantee that the page will
+    // always be displayed using the theme whose blocks are being configured.
+    if ($request->attributes->get(RouteObjectInterface::ROUTE_NAME) == 'block.admin_demo') {
+      return $request->attributes->get('theme');
+    }
+  }
+
+} 
diff --git a/core/modules/contextual/contextual.module b/core/modules/contextual/contextual.module
index 529fa5c..508b9ae 100644
--- a/core/modules/contextual/contextual.module
+++ b/core/modules/contextual/contextual.module
@@ -6,21 +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',
-    'type' => MENU_CALLBACK,
-  );
-
-  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 8ab2f28..6947f6a 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:
+    _ajax_base_page_theme: 'TRUE'
   requirements:
     _permission: 'access contextual links'
diff --git a/core/modules/edit/edit.module b/core/modules/edit/edit.module
index 000f67f..fe4ee15 100644
--- a/core/modules/edit/edit.module
+++ b/core/modules/edit/edit.module
@@ -18,26 +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',
-    '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().
  */
 function edit_permission() {
diff --git a/core/modules/edit/edit.routing.yml b/core/modules/edit/edit.routing.yml
index 27bced7..a0ecec2 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:
+    _ajax_base_page_theme: 'TRUE'
   requirements:
     _permission: 'access in-place editing'
 
@@ -18,6 +20,7 @@ edit.field_form:
     _controller: '\Drupal\edit\EditController::fieldForm'
   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/editor/editor.module b/core/modules/editor/editor.module
index 8282d07..377a79e 100644
--- a/core/modules/editor/editor.module
+++ b/core/modules/editor/editor.module
@@ -140,21 +140,6 @@ 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',
-    'type' => MENU_CALLBACK,
-  );
-
-  return $items;
-}
-
-/**
  * Implements hook_form_FORM_ID_alter().
  */
 function editor_form_filter_admin_overview_alter(&$form, $form_state) {
diff --git a/core/modules/editor/editor.routing.yml b/core/modules/editor/editor.routing.yml
index 46b4fba..4ad64e3 100644
--- a/core/modules/editor/editor.routing.yml
+++ b/core/modules/editor/editor.routing.yml
@@ -4,6 +4,7 @@ editor.field_untransformed_text:
     _controller: '\Drupal\editor\EditorController::getUntransformedText'
   options:
     _access_mode: 'ALL'
+    _ajax_base_page_theme: 'TRUE'
   requirements:
     _permission: 'access in-place editing'
     _access_edit_entity_field: 'TRUE'
diff --git a/core/modules/file/file.module b/core/modules/file/file.module
index 3dcbcef..a16aa09 100644
--- a/core/modules/file/file.module
+++ b/core/modules/file/file.module
@@ -38,21 +38,6 @@ function file_help($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().
  *
  * 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 8bf971c..2b84612 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:
+    _ajax_base_page_theme: 'TRUE'
   requirements:
     _permission: 'access content'
 
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 0000000..e6694e9
--- /dev/null
+++ b/core/modules/system/lib/Drupal/system/Theme/BatchNegotiator.php
@@ -0,0 +1,54 @@
+<?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 determineActiveTheme(Request $request) {
+    if ($request->attributes->get(RouteObjectInterface::ROUTE_NAME) == 'system.batch_page') {
+      // 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.module b/core/modules/system/system.module
index 8437e4c..6fc8da3 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -608,12 +608,6 @@ 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',
@@ -878,13 +872,6 @@ 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,
-  );
-
   // Localize date formats.
   if (module_exists('language')) {
     $items['admin/config/regional/date-time/locale'] = array(
@@ -958,21 +945,6 @@ function system_theme_suggestions_region(array $variables) {
 }
 
 /**
- * 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().
  */
 function system_library_info() {
diff --git a/core/modules/system/system.routing.yml b/core/modules/system/system.routing.yml
index 508d59c..ecb9864 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:
+    _ajax_base_page_theme: 'TRUE'
   requirements:
     _access: 'TRUE'
 
diff --git a/core/modules/system/system.services.yml b/core/modules/system/system.services.yml
index ae137da..d05632d 100644
--- a/core/modules/system/system.services.yml
+++ b/core/modules/system/system.services.yml
@@ -23,3 +23,8 @@ services:
     class: Drupal\system\Routing\RouteSubscriber
     tags:
       - { name: event_subscriber }
+  theme.negotiator.system.batch:
+    class: Drupal\system\Theme\BatchNegotiator
+    arguments: ['@batch.storage']
+    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 0000000..6ca1b71
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/Theme/CustomThemeNegotiator.php
@@ -0,0 +1,29 @@
+<?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 determineActiveTheme(Request $request) {
+    if (($route_object = $request->attributes->get(RouteObjectInterface::ROUTE_OBJECT)) && $route_object instanceof Route && $route_object->hasOption('_custom_theme')) {
+      return $route_object->getOption('_custom_theme');
+    }
+  }
+
+} 
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 7c205c8..2036faf 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -57,26 +57,6 @@ function theme_test_system_theme_info() {
 }
 
 /**
- * Implements hook_menu().
- */
-function theme_test_menu() {
-  $items['theme-test/suggestion'] = array(
-    'route_name' => 'theme_test.suggestion',
-    'theme callback' => '_theme_custom_theme',
-    'type' => MENU_CALLBACK,
-  );
-  $items['theme-test/alter'] = array(
-    'theme callback' => '_theme_custom_theme',
-    'route_name' => 'theme_test.alter',
-    'type' => MENU_CALLBACK,
-  );
-  $items['theme-test/function-template-overridden'] = array(
-    'theme callback' => '_theme_custom_theme',
-    'route_name' => 'theme_test.function_template_override',
-  );
-  return $items;
-}
-/**
  * Fake registry loading callback.
  */
 function _theme_test_load_registry() {
@@ -85,13 +65,6 @@ function _theme_test_load_registry() {
 }
 
 /**
- * Custom theme callback.
- */
-function _theme_custom_theme() {
-  return 'test_theme';
-}
-
-/**
  * Implements hook_preprocess_HOOK() for html.tpl.php.
  */
 function theme_test_preprocess_html(&$variables) {
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 b4a7fd6..8a750f3 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 +23,8 @@ 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'
@@ -29,6 +33,8 @@ 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 8e442df..29c419d 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 +3,8 @@ 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 }
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 $user->theme;
+      }
+    }
+  }
+
+}
diff --git a/core/modules/user/user.services.yml b/core/modules/user/user.services.yml
index 6fb7d47..c70cad1 100644
--- a/core/modules/user/user.services.yml
+++ b/core/modules/user/user.services.yml
@@ -25,3 +25,9 @@ services:
     class: Drupal\user\EventSubscriber\MaintenanceModeSubscriber
     tags:
       - { name: event_subscriber }
+  theme.negotiator.user:
+    class: Drupal\user\Theme\UserNegotiator
+    arguments: ['@plugin.manager.entity', '@current_user']
+    tags:
+      - { name: theme_negotiator, priority: 50 }
+
diff --git a/core/modules/views/lib/Drupal/views/ViewExecutable.php b/core/modules/views/lib/Drupal/views/ViewExecutable.php
index d8236cf..37329c7 100644
--- a/core/modules/views/lib/Drupal/views/ViewExecutable.php
+++ b/core/modules/views/lib/Drupal/views/ViewExecutable.php
@@ -1302,8 +1302,6 @@ public function render($display_id = NULL) {
       return;
     }
 
-    drupal_theme_initialize();
-
     $exposed_form = $this->display_handler->getPlugin('exposed_form');
     $exposed_form->preRender($this->result);
 
@@ -1365,11 +1363,13 @@ 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) {
@@ -1387,11 +1387,13 @@ 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 7a6b346..2525c8e 100644
--- a/core/modules/views/views.module
+++ b/core/modules/views/views.module
@@ -329,20 +329,6 @@ 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 $items;
-}
-
-/**
  * Implement hook_menu_alter().
  */
 function views_menu_alter(&$callbacks) {
@@ -706,6 +692,9 @@ function views_invalidate_cache() {
 
   $module_handler = \Drupal::moduleHandler();
 
+  // Reset the RouteSubscriber from views.
+  \Drupal::getContainer()->get('views.route_subscriber')->reset();
+
   // Set the router to be rebuild.
   // @todo Figure out why the cache rebuild is trigged but the route table
   //   does not exist yet.
diff --git a/core/modules/views/views.routing.yml b/core/modules/views/views.routing.yml
index 9abe5d5..83f2aad 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:
+    _ajax_base_page_theme: 'TRUE'
   requirements:
     _access: 'TRUE'