diff -u b/core/modules/system/src/Tests/Theme/EngineTwigTest.php b/core/modules/system/src/Tests/Theme/EngineTwigTest.php
--- b/core/modules/system/src/Tests/Theme/EngineTwigTest.php
+++ b/core/modules/system/src/Tests/Theme/EngineTwigTest.php
@@ -116,7 +116,8 @@
    */
   public function testActiveTheme() {
     $this->drupalGet('/twig-theme-test/active_theme');
-    $this->assertRaw('<div>active theme: classy</div>');
+    $theme = \Drupal::service('theme.manager')->getActiveTheme()->getName();
+    $this->assertRaw('<div>active theme: ' . $theme . '</div>');
   }
 
 }