diff --git a/core/includes/common.inc b/core/includes/common.inc
index d18c0c1..4b3ba50 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -2253,7 +2253,7 @@ function drupal_get_js($scope = 'header', $javascript = NULL, $skip_alter = FALS
     // Don't add settings if there is no other JavaScript on the page, unless
     // this is an AJAX request.
     if (!empty($items['settings']) || $is_ajax) {
-      global $theme_key;
+      $theme_key = \Drupal::themeNegotiator()->getActiveTheme();
       // Provide the page with information about the theme that's used, so that
       // a later AJAX request can be rendered using the same theme.
       // @see \Drupal\Core\Theme\AjaxBasePageNegotiator
@@ -4560,9 +4560,9 @@ function drupal_render_cache_by_query($query, $function, $expire = Cache::PERMAN
  *   $granularity was passed in, more parts are added.
  */
 function drupal_render_cid_parts($granularity = NULL) {
-  global $theme, $base_root;
+  global $base_root;
 
-  $cid_parts[] = $theme;
+  $cid_parts[] = \Drupal::themeNegotiator()->getActiveTheme();
 
   // If we have only one language enabled we do not need it as cid part.
   $language_manager = \Drupal::languageManager();
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index cfd8427..cdb9f80 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -739,7 +739,7 @@ function drupal_find_theme_templates($cache, $extension, $path) {
       }
     }
   }
-  global $theme;
+  $theme = \Drupal::themeNegotiator()->getActiveTheme();
   $subtheme_paths = isset($theme_paths[$theme]) ? $theme_paths[$theme] : array();
 
   // Escape the periods in the extension.
@@ -841,7 +841,7 @@ function theme_get_setting($setting_name, $theme = NULL) {
 
   // If no key is given, use the current theme if we can determine it.
   if (!isset($theme)) {
-    $theme = !empty($GLOBALS['theme_key']) ? $GLOBALS['theme_key'] : '';
+    $theme = \Drupal::themeNegotiator()->getActiveTheme();
   }
 
   if (empty($cache[$theme])) {
@@ -2247,7 +2247,8 @@ function template_preprocess_maintenance_page(&$variables) {
     drupal_render($build);
   }
 
-  foreach (system_region_list($GLOBALS['theme']) as $region_key => $region_name) {
+  $theme = \Drupal::themeNegotiator()->getActiveTheme();
+  foreach (system_region_list($theme) as $region_key => $region_name) {
     if (!isset($variables[$region_key])) {
       $variables[$region_key] = array();
     }
diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index 429b932..587b66e 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -15,14 +15,19 @@
  * complete. Seven is always used for the initial install and update
  * operations. In other cases, Bartik is used, but this can be overridden by
  * setting a "maintenance_theme" key in the $settings variable in settings.php.
+ *
+ * @todo Figure out whether this logic could be put into a theme negotiator
+ *   itself.
  */
 function _drupal_maintenance_theme() {
-  global $theme, $theme_key;
-
   // If $theme is already set, assume the others are set too, and do nothing.
-  if (isset($theme)) {
+  if (\Drupal::themeNegotiator()->hasActiveTheme()) {
     return;
   }
+  else {
+    // @todo figure out the difference between theme and theme_key.
+    $theme = \Drupal::themeNegotiator()->getActiveTheme();
+  }
 
   require_once DRUPAL_ROOT . '/' . settings()->get('path_inc', 'core/includes/path.inc');
   require_once __DIR__ . '/theme.inc';
diff --git a/core/lib/Drupal.php b/core/lib/Drupal.php
index a646096..00b14dd 100644
--- a/core/lib/Drupal.php
+++ b/core/lib/Drupal.php
@@ -403,6 +403,15 @@ public static function moduleHandler() {
   }
 
   /**
+   * Returns the theme negotiator.
+   *
+   * @return \Drupal\Core\Theme\ThemeNegotiator
+   */
+  public static function themeNegotiator() {
+    return static::$container->get('theme.negotiator');
+  }
+
+  /**
    * Returns the typed data manager service.
    *
    * Use the typed data manager service for creating typed data objects.
diff --git a/core/lib/Drupal/Core/Theme/ThemeNegotiator.php b/core/lib/Drupal/Core/Theme/ThemeNegotiator.php
index 21ba80c..285f471 100644
--- a/core/lib/Drupal/Core/Theme/ThemeNegotiator.php
+++ b/core/lib/Drupal/Core/Theme/ThemeNegotiator.php
@@ -118,6 +118,23 @@ public function getActiveTheme() {
   }
 
   /**
+   * Is there already an active theme.
+   */
+  public function hasActiveTheme() {
+    return $this->request->attributes->has('_theme_active');
+  }
+
+  /**
+   * Sets the current active theme for the request.
+   *
+   * @param string $theme
+   *   The theme name.
+   */
+  public function setActiveTheme($theme) {
+    $this->request->attributes->set('_theme_active', $theme);
+  }
+
+  /**
    * {@inheritdoc}
    */
   public function applies(Request $request) {
diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index 1bca18b..4af3f42 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -112,10 +112,9 @@ function block_menu_link_defaults() {
  * Renders blocks into their regions.
  */
 function block_page_build(&$page) {
-  global $theme;
-
   // The theme system might not yet be initialized. We need $theme.
   drupal_theme_initialize();
+  $theme = \Drupal::themeNegotiator()->getActiveTheme();
 
   // Fetch a list of regions for the current theme.
   $all_regions = system_region_list($theme);
@@ -326,7 +325,7 @@ function block_list($region) {
   $blocks = &drupal_static(__FUNCTION__);
 
   if (!isset($blocks)) {
-    global $theme;
+    $theme = \Drupal::themeNegotiator()->getActiveTheme();
     $blocks = array();
     foreach (entity_load_multiple_by_properties('block', array('theme' => $theme)) as $block_id => $block) {
       // Onlye include valid blocks in the list.
diff --git a/core/modules/block/lib/Drupal/block/BlockListController.php b/core/modules/block/lib/Drupal/block/BlockListController.php
index 65db261..d6b271f 100644
--- a/core/modules/block/lib/Drupal/block/BlockListController.php
+++ b/core/modules/block/lib/Drupal/block/BlockListController.php
@@ -87,7 +87,7 @@ public static function createInstance(ContainerInterface $container, EntityTypeI
   public function load() {
     // If no theme was specified, use the current theme.
     if (!$this->theme) {
-      $this->theme = $GLOBALS['theme'];
+      $this->theme = \Drupal::themeNegotiator()->getActiveTheme();
     }
 
     // Store the region list.
@@ -117,7 +117,7 @@ public function load() {
   public function render($theme = NULL, Request $request = NULL) {
     $this->request = $request;
     // If no theme was specified, use the current theme.
-    $this->theme = $theme ?: $GLOBALS['theme_key'];
+    $this->theme = $theme ?: \Drupal::themeNegotiator()->getActiveTheme();;
 
     return drupal_get_form($this);
   }
diff --git a/core/modules/color/color.module b/core/modules/color/color.module
index 7809f5f..f9da95a 100644
--- a/core/modules/color/color.module
+++ b/core/modules/color/color.module
@@ -62,7 +62,7 @@ function color_form_system_theme_settings_alter(&$form, &$form_state) {
  * Replaces style sheets with color-altered style sheets.
  */
 function color_css_alter(&$css) {
-  global $theme_key;
+  $theme_key =  \Drupal::themeNegotiator()->getActiveTheme();
   $themes = list_themes();
 
   // Override stylesheets.
@@ -92,7 +92,7 @@ function color_css_alter(&$css) {
  * Replace the logo with the colored version if available.
  */
 function color_preprocess_page(&$variables) {
-  global $theme_key;
+  $theme_key = \Drupal::themeNegotiator()->getActiveTheme();
 
   // Override logo.
   $logo = \Drupal::config('color.' . $theme_key)->get('logo');
diff --git a/core/modules/system/lib/Drupal/system/Form/ThemeSettingsForm.php b/core/modules/system/lib/Drupal/system/Form/ThemeSettingsForm.php
index 9459aad..137bd64 100644
--- a/core/modules/system/lib/Drupal/system/Form/ThemeSettingsForm.php
+++ b/core/modules/system/lib/Drupal/system/Form/ThemeSettingsForm.php
@@ -282,8 +282,8 @@ public function buildForm(array $form, array &$form_state, $theme = '') {
       // Save the name of the current theme (if any), so that we can temporarily
       // override the current theme and allow theme_get_setting() to work
       // without having to pass the theme name to it.
-      $default_theme = !empty($GLOBALS['theme_key']) ? $GLOBALS['theme_key'] : NULL;
-      $GLOBALS['theme_key'] = $theme;
+      $default_theme = \Drupal::themeNegotiator()->getActiveTheme();
+      \Drupal::themeNegotiator()->setActiveTheme($theme);
 
       // Process the theme and all its base themes.
       foreach ($theme_keys as $theme) {
@@ -302,10 +302,10 @@ public function buildForm(array $form, array &$form_state, $theme = '') {
 
       // Restore the original current theme.
       if (isset($default_theme)) {
-        $GLOBALS['theme_key'] = $default_theme;
+        \Drupal::themeNegotiator()->setActiveTheme($default_theme);
       }
       else {
-        unset($GLOBALS['theme_key']);
+        \Drupal::themeNegotiator()->setActiveTheme(NULL);
       }
     }
 
diff --git a/core/modules/system/lib/Drupal/system/Tests/Common/AlterTest.php b/core/modules/system/lib/Drupal/system/Tests/Common/AlterTest.php
index e02c590..e249652 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/AlterTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/AlterTest.php
@@ -35,8 +35,8 @@ public static function getInfo() {
   function testDrupalAlter() {
     // This test depends on Bartik, so make sure that it is always the current
     // active theme.
-    global $theme, $base_theme_info;
-    $theme = 'bartik';
+    global $base_theme_info;
+    \Drupal::themeNegotiator()->setActiveTheme('bartik');
     $base_theme_info = array();
 
     $array = array('foo' => 'bar');
diff --git a/core/modules/system/lib/Drupal/system/Tests/Common/RegionContentTest.php b/core/modules/system/lib/Drupal/system/Tests/Common/RegionContentTest.php
index e8b8201..6450010 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/RegionContentTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/RegionContentTest.php
@@ -25,7 +25,7 @@ public static function getInfo() {
    * Tests setting and retrieving content for theme regions.
    */
   function testRegions() {
-    global $theme_key;
+    $theme_key = \Drupal::themeNegotiator()->getActiveTheme();
 
     $block_regions = array_keys(system_region_list($theme_key));
     $delimiter = $this->randomName(32);
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 638d8e5..1f08a89 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -223,7 +223,7 @@ function testListThemes() {
    * Test the theme_get_setting() function.
    */
   function testThemeGetSetting() {
-    $GLOBALS['theme_key'] = 'test_theme';
+    \Drupal::themeNegotiator()->setActiveTheme('test_theme');
     $this->assertIdentical(theme_get_setting('theme_test_setting'), 'default value', 'theme_get_setting() uses the default theme automatically.');
     $this->assertNotEqual(theme_get_setting('subtheme_override', 'test_basetheme'), theme_get_setting('subtheme_override', 'test_subtheme'), 'Base theme\'s default settings values can be overridden by subtheme.');
     $this->assertIdentical(theme_get_setting('basetheme_only', 'test_subtheme'), 'base theme value', 'Base theme\'s default settings values are inherited by subtheme.');
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 65a8796..bbbc177 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -1896,7 +1896,7 @@ function system_retrieve_file($url, $destination = NULL, $managed = FALSE, $repl
 function system_page_alter(&$page) {
   // Find all non-empty page regions, and add a theme wrapper function that
   // allows them to be consistently themed.
-  $regions = system_region_list($GLOBALS['theme']);
+  $regions = system_region_list(\Drupal::themeNegotiator()->getActiveTheme());
   foreach (array_keys($regions) as $region) {
     if (!empty($page[$region])) {
       $page[$region]['#theme_wrappers'][] = 'region';
diff --git a/core/modules/system/tests/modules/batch_test/batch_test.module b/core/modules/system/tests/modules/batch_test/batch_test.module
index b97f966..96d39c2 100644
--- a/core/modules/system/tests/modules/batch_test/batch_test.module
+++ b/core/modules/system/tests/modules/batch_test/batch_test.module
@@ -374,8 +374,7 @@ function _batch_test_theme_callback() {
   // theme is being used on the batch processing page by viewing that page
   // directly. Instead, we save the theme being used in a variable here, so
   // that it can be loaded and inspected in the thread running the test.
-  global $theme;
-  batch_test_stack($theme);
+  batch_test_stack(\Drupal::themeNegotiator()->getActiveTheme());
 }
 
 /**
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 08e6382..aecd648 100644
--- a/core/modules/system/tests/modules/menu_test/menu_test.module
+++ b/core/modules/system/tests/modules/menu_test/menu_test.module
@@ -262,9 +262,9 @@ function menu_test_custom_403_404_callback() {
  * @deprecated Use \Drupal\menu_test\Controller\MenuTestController::themePage()
  */
 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();
+  $theme_key = \Drupal::themeNegotiator()->getActiveTheme();
   // 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.";
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/cache/CachePluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/cache/CachePluginBase.php
index 665b7a5..2d1e0d0 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/cache/CachePluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/cache/CachePluginBase.php
@@ -314,7 +314,7 @@ public function generateOutputKey() {
         'result' => $this->view->result,
         'roles' => $user->getRoles(),
         'super-user' => $user->id() == 1, // special caching for super user.
-        'theme' => $GLOBALS['theme'],
+        'theme' => \Drupal::themeNegotiator()->getActiveTheme(),
         'langcode' => \Drupal::languageManager()->getCurrentLanguage()->id,
         'base_url' => $GLOBALS['base_url'],
       );