reverted: --- b/core/modules/system/src/Tests/Common/RenderTest.php +++ a/core/modules/system/src/Tests/Common/RenderTest.php @@ -581,10 +581,6 @@ $request_method = \Drupal::request()->getMethod(); \Drupal::request()->setMethod('GET'); - // Enable the Classy theme. - \Drupal::service('theme_handler')->install(['classy']); - $this->config('system.theme')->set('default', 'classy')->save(); - // Test case 1. // Create an element with a child and subchild. Each element has the same // #post_render_cache callback, but with different contexts.