diff -u b/core/lib/Drupal/Core/Theme/ThemeNegotiatorInterface.php b/core/lib/Drupal/Core/Theme/ThemeNegotiatorInterface.php
--- b/core/lib/Drupal/Core/Theme/ThemeNegotiatorInterface.php
+++ b/core/lib/Drupal/Core/Theme/ThemeNegotiatorInterface.php
@@ -11,6 +11,12 @@
 
 /**
  * Defines an interface for classes which determine the active theme.
+ *
+ * In order to set the active theme you have to create a new service tagged
+ * with 'theme_negotiator', see user.services.yml as example. The only method
+ * this service has to implement is determineActiveTheme. Return the name of the
+ * theme or NULL, if other negotiators like the configured default one should
+ * kick in instead.
  */
 interface ThemeNegotiatorInterface {
 
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/Theme/HighPriorityThemeNegotiator.php
@@ -0,0 +1,28 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\theme_test\Theme\HighPriorityThemeNegotiator.
+ */
+
+namespace Drupal\theme_test\Theme;
+
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
+use Symfony\Cmf\Component\Routing\RouteObjectInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Implements a test theme negotiator which was configured with a high priority.
+ */
+class HighPriorityThemeNegotiator implements ThemeNegotiatorInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(Request $request) {
+    if (($route_name = $request->attributes->get(RouteObjectInterface::ROUTE_NAME)) && $route_name == 'theme_test.priority') {
+      return 'stark';
+    }
+  }
+
+}