diff -u b/core/modules/system/lib/Drupal/system/Tests/Menu/RouterTest.php b/core/modules/system/lib/Drupal/system/Tests/Menu/RouterTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/Menu/RouterTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Menu/RouterTest.php
@@ -29,7 +29,7 @@
     // Make the tests below more robust by explicitly setting the default theme
     // and administrative theme that they expect.
     theme_enable(array('bartik'));
-    $config
+    config('system.theme')
       ->set('default', 'bartik')
       ->set('admin', 'seven')
       ->save();