diff --git a/core/config/install/core.extension.yml b/core/config/install/core.extension.yml
index debc391..eae39ef 100644
--- a/core/config/install/core.extension.yml
+++ b/core/config/install/core.extension.yml
@@ -1,6 +1,5 @@
 module: {}
 theme:
-  seven: 0
   stark: 0
 disabled:
   theme: {}
diff --git a/core/includes/common.inc b/core/includes/common.inc
index 9bfc20a..73e2c4c 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -4058,7 +4058,7 @@ function drupal_flush_all_caches() {
   // use it. Unlike regular usages of this function, the installer and update
   // scripts need to flush all caches during GET requests/page building.
   if (function_exists('_drupal_maintenance_theme')) {
-    unset($GLOBALS['theme']);
+    \Drupal::theme()->resetActiveTheme();
     drupal_maintenance_theme();
   }
 }
diff --git a/core/includes/errors.inc b/core/includes/errors.inc
index bb5448b..51add03 100644
--- a/core/includes/errors.inc
+++ b/core/includes/errors.inc
@@ -126,12 +126,11 @@ function _drupal_log_error($error, $fatal = FALSE) {
     // point in time already. Do not unset that.
     if (!$is_installer) {
       \Drupal::theme()->setActiveTheme(NULL);
-      unset($GLOBALS['theme']);
     }
     if (!defined('MAINTENANCE_MODE')) {
       define('MAINTENANCE_MODE', 'error');
     }
-    // No-op if $GLOBALS['theme'] is set already.
+    // No-op if the active theme is set already.
     drupal_maintenance_theme();
   }
 
diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index f9c41c3..2adb930 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -19,10 +19,8 @@
  * setting a "maintenance_theme" key in the $settings variable in settings.php.
  */
 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 the theme is already set, assume the others are set too, and do nothing.
+  if (\Drupal::theme()->hasActiveTheme()) {
     return;
   }
 
@@ -92,9 +90,6 @@ function _drupal_maintenance_theme() {
   // list_themes() builds its cache.
   $theme = $custom_theme;
 
-  // Store the identifier for retrieving theme settings with.
-  $theme_key = $theme;
-
   // Find all our ancestor themes and put them in an array.
   $base_theme = array();
   $ancestor = $theme;
diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index 5c4ce84..de41361 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -76,6 +76,20 @@ public function getActiveTheme(RouteMatchInterface $route_match = NULL) {
   /**
    * {@inheritdoc}
    */
+  public function hasActiveTheme() {
+    return isset($this->activeTheme);
+  }
+
+  /**
+   * @TODO
+   */
+  public function resetActiveTheme() {
+    $this->activeTheme = NULL;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function setActiveTheme(ActiveTheme $active_theme) {
     $this->activeTheme = $active_theme;
     if ($active_theme) {
diff --git a/core/lib/Drupal/Core/Theme/ThemeManagerInterface.php b/core/lib/Drupal/Core/Theme/ThemeManagerInterface.php
index e3d851f..0d11a39 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManagerInterface.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManagerInterface.php
@@ -33,6 +33,16 @@ public function render($hook, $variables);
    */
   public function getActiveTheme();
 
+  /**
+   * @TODO
+   */
+  public function hasActiveTheme();
+
+  /**
+   * @TODO
+   */
+  public function resetActiveTheme();
+
   public function setActiveTheme(ActiveTheme $active_theme);
 
   /**
diff --git a/core/modules/block/src/BlockListBuilder.php b/core/modules/block/src/BlockListBuilder.php
index 1b26b9f..dd171f9 100644
--- a/core/modules/block/src/BlockListBuilder.php
+++ b/core/modules/block/src/BlockListBuilder.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::theme()->getActiveTheme()->getName();
     }
 
     // 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::theme()->getActiveTheme()->getName();
 
     return \Drupal::formBuilder()->getForm($this);
   }
diff --git a/core/modules/color/color.module b/core/modules/color/color.module
index 3577a8d..663a92c 100644
--- a/core/modules/color/color.module
+++ b/core/modules/color/color.module
@@ -97,7 +97,7 @@ function color_css_alter(&$css) {
  * Replace the logo with the colored version if available.
  */
 function color_preprocess_page(&$variables) {
-  global $theme_key;
+  $theme_key = \Drupal::theme()->getActiveTheme()->getName();
 
   // Override logo.
   $logo = \Drupal::config('color.theme.' . $theme_key)->get('logo');
diff --git a/core/modules/simpletest/src/TestBase.php b/core/modules/simpletest/src/TestBase.php
index afe1feb..4e89fd6 100644
--- a/core/modules/simpletest/src/TestBase.php
+++ b/core/modules/simpletest/src/TestBase.php
@@ -1080,12 +1080,6 @@ private function prepareEnvironment() {
     unset($GLOBALS['config_directories']);
     unset($GLOBALS['config']);
     unset($GLOBALS['conf']);
-    unset($GLOBALS['theme_key']);
-    unset($GLOBALS['theme']);
-    unset($GLOBALS['theme_info']);
-    unset($GLOBALS['base_theme_info']);
-    unset($GLOBALS['theme_engine']);
-    unset($GLOBALS['theme_path']);
 
     // Log fatal errors.
     ini_set('log_errors', 1);
@@ -1186,14 +1180,6 @@ private function restoreEnvironment() {
     // this second reset is guaranteed to reset everything to nothing.
     drupal_static_reset();
 
-    // Reset global theme variables.
-    unset($GLOBALS['theme_key']);
-    unset($GLOBALS['theme']);
-    unset($GLOBALS['theme_info']);
-    unset($GLOBALS['base_theme_info']);
-    unset($GLOBALS['theme_engine']);
-    unset($GLOBALS['theme_path']);
-
     // Restore original in-memory configuration.
     $GLOBALS['config'] = $this->originalConfig;
     $GLOBALS['conf'] = $this->originalConf;
diff --git a/core/modules/system/src/Form/ThemeSettingsForm.php b/core/modules/system/src/Form/ThemeSettingsForm.php
index af52b04..ceaf612 100644
--- a/core/modules/system/src/Form/ThemeSettingsForm.php
+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -284,8 +284,11 @@ public function buildForm(array $form, FormStateInterface $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_active_theme = \Drupal::theme()->getActiveTheme();
+      $default_theme = $default_active_theme->getName();
+      /** @var \Drupal\Core\Theme\Initialization $theme_initialization */
+      $theme_initialization = \Drupal::service('theme.initialization');
+      \Drupal::theme()->setActiveTheme($theme_initialization->getActiveThemeByName($theme));
 
       // Process the theme and all its base themes.
       foreach ($theme_keys as $theme) {
@@ -304,10 +307,10 @@ public function buildForm(array $form, FormStateInterface $form_state, $theme =
 
       // Restore the original current theme.
       if (isset($default_theme)) {
-        $GLOBALS['theme_key'] = $default_theme;
+        \Drupal::theme()->setActiveTheme($default_theme);
       }
       else {
-        unset($GLOBALS['theme_key']);
+        \Drupal::theme()->setActiveTheme(NULL);
       }
     }
 
diff --git a/core/modules/system/src/Tests/Common/RegionContentTest.php b/core/modules/system/src/Tests/Common/RegionContentTest.php
index 6b2d3a4..51ea973 100644
--- a/core/modules/system/src/Tests/Common/RegionContentTest.php
+++ b/core/modules/system/src/Tests/Common/RegionContentTest.php
@@ -19,7 +19,7 @@ class RegionContentTest extends WebTestBase {
    * Tests setting and retrieving content for theme regions.
    */
   function testRegions() {
-    global $theme_key;
+    $theme_key = \Drupal::theme()->getActiveTheme()->getName();
 
     $block_regions = array_keys(system_region_list($theme_key));
     $delimiter = $this->randomName(32);
diff --git a/core/modules/views/src/Plugin/views/cache/CachePluginBase.php b/core/modules/views/src/Plugin/views/cache/CachePluginBase.php
index 08c07b7..ff02186 100644
--- a/core/modules/views/src/Plugin/views/cache/CachePluginBase.php
+++ b/core/modules/views/src/Plugin/views/cache/CachePluginBase.php
@@ -328,7 +328,7 @@ 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::theme()->getActiveTheme()->getName(),
         'langcode' => \Drupal::languageManager()->getCurrentLanguage()->id,
         'base_url' => $GLOBALS['base_url'],
       );