diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 805457c..3d1da49 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -85,7 +85,7 @@ function drupal_theme_access($theme) {
  * Initializes the theme system by loading the theme.
  */
 function drupal_theme_initialize() {
-  global $theme, $user, $theme_key;
+  global $theme, $theme_key;
 
   // If $theme is already set, assume the others are set, too, and do nothing
   if (isset($theme)) {
diff --git a/core/lib/Drupal/Core/CoreServiceProvider.php b/core/lib/Drupal/Core/CoreServiceProvider.php
index 4bd88ee..12231f2 100644
--- a/core/lib/Drupal/Core/CoreServiceProvider.php
+++ b/core/lib/Drupal/Core/CoreServiceProvider.php
@@ -93,13 +93,15 @@ protected function registerModuleHandler(ContainerBuilder $container) {
     if ($container->getParameter('kernel.environment') == 'install') {
       // During installation we use the non-cached version.
       $container->register('module_handler', 'Drupal\Core\Extension\ModuleHandler')
-        ->addArgument('%container.modules%');
+        ->addArgument('%container.modules%')
+        ->addMethodCall('setThemeNegotiator', array(new Reference('theme.negotiator')));
     }
     else {
       $container->register('module_handler', 'Drupal\Core\Extension\CachedModuleHandler')
         ->addArgument('%container.modules%')
         ->addArgument(new Reference('state'))
-        ->addArgument(new Reference('cache.bootstrap'));
+        ->addArgument(new Reference('cache.bootstrap'))
+        ->addMethodCall('setThemeNegotiator', array(new Reference('theme.negotiator')));
     }
   }
 
diff --git a/core/lib/Drupal/Core/Extension/ModuleHandler.php b/core/lib/Drupal/Core/Extension/ModuleHandler.php
index 92cf2a4..2f35df1 100644
--- a/core/lib/Drupal/Core/Extension/ModuleHandler.php
+++ b/core/lib/Drupal/Core/Extension/ModuleHandler.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Extension;
 
 use Drupal\Component\Graph\Graph;
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
 use Symfony\Component\Yaml\Parser;
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Cache\CacheBackendInterface;
@@ -65,6 +66,11 @@ class ModuleHandler implements ModuleHandlerInterface {
   protected $alterFunctions;
 
   /**
+   * @var \Drupal\Core\Theme\ThemeNegotiatorInterface
+   */
+  protected $themeNegotiator;
+
+  /**
    * Constructs a ModuleHandler object.
    *
    * @param array $module_list
@@ -79,6 +85,10 @@ public function __construct(array $module_list = array()) {
     $this->moduleList = $module_list;
   }
 
+  public function setThemeNegotiator(ThemeNegotiatorInterface $theme_negotiator) {
+    $this->themeNegotiator = $theme_negotiator;
+  }
+
   /**
    * Implements \Drupal\Core\Extension\ModuleHandlerInterface::load().
    */
@@ -378,8 +388,9 @@ public function alter($type, &$data, &$context1 = NULL, &$context2 = NULL) {
       }
       // Allow the theme to alter variables after the theme system has been
       // initialized.
-      global $theme, $base_theme_info;
-      if (isset($theme)) {
+      if ($this->themeNegotiator && $theme = $this->themeNegotiator->determineActiveTheme(\Drupal::request())) {
+        drupal_theme_initialize();
+        global $base_theme_info;
         $theme_keys = array();
         foreach ($base_theme_info as $base) {
           $theme_keys[] = $base->name;
diff --git a/core/modules/user/user.services.yml b/core/modules/user/user.services.yml
index c70cad1..6fb7d47 100644
--- a/core/modules/user/user.services.yml
+++ b/core/modules/user/user.services.yml
@@ -25,9 +25,3 @@ 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 }
-