diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
index 41760ee..aae5fd1 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -289,14 +289,9 @@ function testRegionClass() {
     \Drupal::moduleHandler()->install(array('system', 'block', 'theme_region_test'));
     $this->drupalPlaceBlock('system_powered_by_block');
 
-    // Explicitly set the default and admin themes.
-    $theme = 'test_theme';
-    theme_enable(array($theme));
-    \Drupal::config('system.theme')->set('default', $theme)->save();
     // Place a block.
-    $block = $this->drupalPlaceBlock('system_main_block');
-    \Drupal::service('router.builder')->rebuild();
-    $this->drupalGet('node/1');
+    $this->drupalPlaceBlock('system_main_block');
+    $this->drupalGet('');
     $this->assertRaw('new_class', 'New class found.');
   }
 }
diff --git a/core/modules/system/tests/modules/theme_region_test/theme_region_test.info.yml b/core/modules/system/tests/modules/theme_region_test/theme_region_test.info.yml
index cc1a557..0cf5fe3 100644
--- a/core/modules/system/tests/modules/theme_region_test/theme_region_test.info.yml
+++ b/core/modules/system/tests/modules/theme_region_test/theme_region_test.info.yml
@@ -4,4 +4,3 @@ description: 'Provides hook implementations for testing regions.'
 package: Testing
 version: VERSION
 core: 8.x
-hidden: true
diff --git a/core/modules/system/tests/modules/theme_region_test/theme_region_test.module b/core/modules/system/tests/modules/theme_region_test/theme_region_test.module
index 509c9e4..c69cbe2 100644
--- a/core/modules/system/tests/modules/theme_region_test/theme_region_test.module
+++ b/core/modules/system/tests/modules/theme_region_test/theme_region_test.module
@@ -2,12 +2,12 @@
 
 /**
  * @file
- * Provides System module hook implementations for testing purposes.
+ * Provides hook implementations for testing purposes.
  */
 
 /**
  * Implements hook_preprocess_HOOK() for region templates.
  */
-function theme_test_preprocess_region(&$variables) {
+function theme_region_test_preprocess_region(&$variables) {
   $variables['attributes']['class'][] = 'new_class';
 }