diff -u b/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
--- b/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -577,7 +577,7 @@
   }
 
   /**
-   * This completes the theme registry adding discovered functions and hooks.
+   * Completes the theme registry adding discovered functions and hooks.
    *
    * @param array $cache
    *   The theme registry.
@@ -702,7 +702,7 @@
   }
 
   /**
-   * Get all user functions grouped by the word before the first underscore.
+   * Gets all user functions grouped by the word before the first underscore.
    *
    * @return array
    *   Functions grouped by the first prefix.
diff -u b/core/modules/system/src/Tests/Theme/ThemeTest.php b/core/modules/system/src/Tests/Theme/ThemeTest.php
--- b/core/modules/system/src/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeTest.php
@@ -277,7 +277,7 @@
   /**
    * Tests that region attributes can be manipulated via preprocess functions.
    */
-  function testRegionClass() {
+  public function testRegionClass() {
     \Drupal::service('module_installer')->install(array('block', 'theme_region_test'));
 
     // Place a block.
@@ -288,17 +288,14 @@
   }
 
   /**
-   * Ensures suggestion preprocess functions run even for default
-   * implementations.
+   * Ensures suggestion preprocess functions run for default implementations.
    *
    * The theme hook used by this test has its base preprocess function in a
    * separate file, so this test also ensures that that file is correctly loaded
    * when needed.
    */
-  function testSuggestionPreprocessForDefaults() {
-    $this->config('system.theme')
-      ->set('default', 'test_theme')
-      ->save();
+  public function testSuggestionPreprocessForDefaults() {
+    \Drupal::service('theme_handler')->setDefault('test_theme');
     // Test with both an unprimed and primed theme registry.
     drupal_theme_rebuild();
     for ($i = 0; $i < 2; $i++) {
@@ -307,6 +304,7 @@
       $expected_values = [
         'Suggestion',
         'Kitten',
+        'Monkey',
         'Kitten',
         'Flamingo',
       ];
diff -u b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
--- b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
+++ b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
@@ -144,10 +144,9 @@
   }
 
   /**
-   * Menu callback for testing preprocess functions are being run for theme
-   * suggestions.
+   * Controller for testing preprocess functions with theme suggestions.
    */
-  function preprocessSuggestions() {
+  public function preprocessSuggestions() {
     return [
       [
         '#theme' => 'theme_test_preprocess_suggestions',
@@ -157,6 +156,10 @@
         '#theme' => 'theme_test_preprocess_suggestions',
         '#foo' => 'kitten',
       ],
+      [
+        '#theme' => 'theme_test_preprocess_suggestions',
+        '#foo' => 'monkey',
+      ],
       ['#theme' => 'theme_test_preprocess_suggestions__kitten__flamingo'],
     ];
   }
diff -u b/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
--- b/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -110,6 +110,14 @@
 }
 
 /**
+ * Tests a module overriding a default hook with a suggestion.
+ */
+function theme_test_preprocess_theme_test_preprocess_suggestions__monkey(&$variables) {
+  $variables['foo'] = 'Monkey';
+}
+
+
+/**
  * Prepares variables for test render element templates.
  *
  * Default template: theme-test-render-element.html.twig.