diff -u b/core/core.services.yml b/core/core.services.yml
--- b/core/core.services.yml
+++ b/core/core.services.yml
@@ -174,7 +174,7 @@
     class: Drupal\Core\Theme\DefaultNegotiator
     arguments: ['@config.factory']
     tags:
-      - { name: theme_negotiator, priority: 100 }
+      - { name: theme_negotiator, priority: -100 }
   theme.negotiator.ajax_base_page:
     class: Drupal\Core\Theme\AjaxBasePageNegotiator
     arguments: ['@csrf_token']
diff -u b/core/includes/theme.inc b/core/includes/theme.inc
--- b/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -94,11 +94,11 @@
 
   drupal_bootstrap(DRUPAL_BOOTSTRAP_DATABASE);
 
-  // Only select the user selected theme if it is available in the
-  // list of themes that can be accessed.
   $themes = list_themes();
 
   // @todo Let the theme.negotiator listen to the kernel request event.
+  // Determine the active theme for the theme negotiator service. This includes
+  // the default theme as well as really specific ones like the ajax base theme.
   $request = \Drupal::request();
   $theme = \Drupal::service('theme.negotiator')->determineActiveTheme($request) ?: 'stark';
 
diff -u b/core/lib/Drupal/Core/EventSubscriber/LegacyRequestSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/LegacyRequestSubscriber.php
--- b/core/lib/Drupal/Core/EventSubscriber/LegacyRequestSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/LegacyRequestSubscriber.php
@@ -54,6 +54,7 @@
    */
   static function getSubscribedEvents() {
     $events[KernelEvents::REQUEST][] = array('onKernelRequestLegacy', 90);
+    // Initialize the theme system after the routing system.
     $events[KernelEvents::REQUEST][] = array('onKernelRequestLegacyAfterRouting', 30);
 
     return $events;
diff -u b/core/lib/Drupal/Core/Theme/ThemeNegotiator.php b/core/lib/Drupal/Core/Theme/ThemeNegotiator.php
--- b/core/lib/Drupal/Core/Theme/ThemeNegotiator.php
+++ b/core/lib/Drupal/Core/Theme/ThemeNegotiator.php
@@ -122,13 +122,12 @@
    */
   public function determineActiveTheme(Request $request) {
     foreach ($this->getSortedNegotiators() as $negotiator) {
-      if (($active_theme = $negotiator->determineActiveTheme($request))) {
-        if ($this->themeAccess->checkAccess($active_theme)) {
-          $request->attributes->set('_theme_active', $active_theme);
-        }
+      $theme = $negotiator->determineActiveTheme($request);
+      if ($theme !== NULL && $this->themeAccess->checkAccess($theme)) {
+        $request->attributes->set('_theme_active', $theme);
+        return $request->attributes->get('_theme_active');
       }
     }
-    return $request->attributes->get('_theme_active');
   }
 
 }
diff -u b/core/modules/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php b/core/modules/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php
@@ -536,16 +536,6 @@
   }
 
   /**
-   * Test that the theme negotiation is properly inherited.
-   */
-  protected function doTestThemeCallbackInheritance() {
-    theme_enable(array($this->admin_theme));
-    $this->drupalGet('menu-test/theme-callback/use-admin-theme/inheritance');
-    $this->assertText('Active theme: seven. Actual theme: seven. Theme negotiation inheritance is being tested.', 'Theme negotiation inheritance correctly uses the administrative theme.');
-    $this->assertRaw('seven/style.css', "The administrative theme's CSS appears on the page.");
-  }
-
-  /**
    * Test the theme negotiation when the site is in maintenance mode.
    */
   protected function doTestThemeCallbackMaintenanceMode() {
diff -u b/core/modules/system/system.services.yml b/core/modules/system/system.services.yml
--- b/core/modules/system/system.services.yml
+++ b/core/modules/system/system.services.yml
@@ -31 +31 @@
-      - { name: theme_negotiator, priority: 40 }
+      - { name: theme_negotiator, priority: -40 }
diff -u b/core/modules/user/user.services.yml b/core/modules/user/user.services.yml
--- b/core/modules/user/user.services.yml
+++ b/core/modules/user/user.services.yml
@@ -31,3 +31,3 @@
     tags:
-      - { name: theme_negotiator, priority: 50 }
+      - { name: theme_negotiator, priority: -50 }
 
diff -u b/core/tests/Drupal/Tests/Core/Theme/ThemeNegotiatorTest.php b/core/tests/Drupal/Tests/Core/Theme/ThemeNegotiatorTest.php
--- b/core/tests/Drupal/Tests/Core/Theme/ThemeNegotiatorTest.php
+++ b/core/tests/Drupal/Tests/Core/Theme/ThemeNegotiatorTest.php
@@ -82,14 +82,13 @@
       ->method('determineActiveTheme')
       ->will($this->returnValue('example_test'));
 
-    $this->themeNegotiator->addNegotiator($negotiator, 0);
+    $this->themeNegotiator->addNegotiator($negotiator, 10);
 
     $negotiator = $this->getMock('Drupal\Core\Theme\ThemeNegotiatorInterface');
-    $negotiator->expects($this->once())
-      ->method('determineActiveTheme')
-      ->will($this->returnValue('example_test2'));
+    $negotiator->expects($this->never())
+      ->method('determineActiveTheme');
 
-    $this->themeNegotiator->addNegotiator($negotiator, 10);
+    $this->themeNegotiator->addNegotiator($negotiator, 0);
 
     $this->themeAccessCheck->expects($this->any())
       ->method('checkAccess')
@@ -113,24 +112,24 @@
       ->method('determineActiveTheme')
       ->will($this->returnValue('example_test'));
 
-    $this->themeNegotiator->addNegotiator($negotiator, 0);
+    $this->themeNegotiator->addNegotiator($negotiator, 10);
 
     $negotiator = $this->getMock('Drupal\Core\Theme\ThemeNegotiatorInterface');
     $negotiator->expects($this->once())
       ->method('determineActiveTheme')
       ->will($this->returnValue('example_test2'));
 
-    $this->themeNegotiator->addNegotiator($negotiator, 10);
+    $this->themeNegotiator->addNegotiator($negotiator, 0);
 
     $this->themeAccessCheck->expects($this->at(0))
       ->method('checkAccess')
-      ->with('example_test2')
-      ->will($this->returnValue(TRUE));
+      ->with('example_test')
+      ->will($this->returnValue(FALSE));
 
     $this->themeAccessCheck->expects($this->at(1))
       ->method('checkAccess')
-      ->with('example_test')
-      ->will($this->returnValue(FALSE));
+      ->with('example_test2')
+      ->will($this->returnValue(TRUE));
 
     $request = Request::create('/test-route');
     $theme = $this->themeNegotiator->determineActiveTheme($request);