diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index cea15c1..7852f93 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -190,7 +190,7 @@ public function buildByExtension($extension) {
             // A regular URI (e.g., http://example.com/example.js) without
             // 'external' explicitly specified, which may happen if, e.g.
             // libraries-override is used.
-            else if ($this->isValidUri($source)) {
+            elseif ($this->isValidUri($source)) {
               $options['type'] = 'external';
               $options['data'] = $source;
             }
@@ -359,7 +359,7 @@ protected function applyLibrariesOverride($libraries, $extension) {
           if (substr_count($asset, '/') < 3) {
             throw new \LogicException(sprintf('Library asset %s is not correctly specified. It should be in the form "extension/library_name/sub_key/path/to/asset.js".', $asset));
           }
-          list(, , $sub_key, $value) = explode('/', $asset, 4);
+          list(,, $sub_key, $value) = explode('/', $asset, 4);
           if ($sub_key === 'drupalSettings') {
             // drupalSettings may not be overridden.
             throw new \LogicException(sprintf('drupalSettings may not be overridden in libraries-override. Trying to override %s. Use hook_library_info_alter() instead.', $asset));
@@ -384,6 +384,7 @@ protected function applyLibrariesOverride($libraries, $extension) {
         }
       }
     }
+
     return $libraries;
   }
 
diff --git a/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
index fee7cd6..e35431d 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -167,7 +167,7 @@ public function getActiveTheme(Extension $theme, array $base_themes = []) {
     // Prepare libraries overrides from this theme and ancestor themes.
     // This allows child themes to easily remove CSS files from base themes and
     // modules.
-    $values['libraries_override'] = array();
+    $values['libraries_override'] = [];
 
     // Get libraries-override declared by base theme.
     foreach ($base_themes as $base) {
@@ -264,7 +264,7 @@ protected function resolveStyleSheetPlaceholders($css_file) {
    * @param \Drupal\Core\Extension\Extension[] $base_themes
    *   An array of base themes.
    *
-   * @return array
+   * @return string
    *
    * @todo Remove in Drupal 9.0.x
    */
diff --git a/core/modules/system/tests/themes/test_theme/css/collapse.css b/core/modules/system/tests/themes/test_theme/css/collapse.css
deleted file mode 100644
index c781141..0000000
--- a/core/modules/system/tests/themes/test_theme/css/collapse.css
+++ /dev/null
@@ -1,4 +0,0 @@
-/**
- * @file
- * Test CSS asset file for test_theme.theme
- */
diff --git a/core/modules/system/tests/themes/test_theme/js/collapse.js b/core/modules/system/tests/themes/test_theme/js/collapse.js
deleted file mode 100644
index bfb509c..0000000
--- a/core/modules/system/tests/themes/test_theme/js/collapse.js
+++ /dev/null
@@ -1,4 +0,0 @@
-/**
- * @file
- * Test JS asset file for test_theme.theme
- */
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml b/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml
deleted file mode 100644
index 69d07fb..0000000
--- a/core/modules/system/tests/themes/test_theme/test_theme.libraries.yml
+++ /dev/null
@@ -1,7 +0,0 @@
-collapse:
-  js:
-    js/collapse.js: { }
-
-  css:
-    base:
-      css/collapse.css: { }
diff --git a/core/modules/system/tests/themes/test_theme_libraries_override_with_drupal_settings/test_theme_libraries_override_with_drupal_settings.info.yml b/core/modules/system/tests/themes/test_theme_libraries_override_with_drupal_settings/test_theme_libraries_override_with_drupal_settings.info.yml
deleted file mode 100644
index 0939c27..0000000
--- a/core/modules/system/tests/themes/test_theme_libraries_override_with_drupal_settings/test_theme_libraries_override_with_drupal_settings.info.yml
+++ /dev/null
@@ -1,9 +0,0 @@
-name: 'Test theme libraries-override'
-type: theme
-description: 'Theme with drupalSettings libraries-override'
-version: VERSION
-base theme: classy
-core: 8.x
-libraries-override:
-  # drupalSettings libraries override. Should throw a \LogicException.
-  core/drupal.ajax/drupalSettings/ajaxPageState: { }
diff --git a/core/modules/system/tests/themes/test_theme_libraries_override_with_invalid_asset/test_theme_libraries_override_with_invalid_asset.info.yml b/core/modules/system/tests/themes/test_theme_libraries_override_with_invalid_asset/test_theme_libraries_override_with_invalid_asset.info.yml
deleted file mode 100644
index ecba74e..0000000
--- a/core/modules/system/tests/themes/test_theme_libraries_override_with_invalid_asset/test_theme_libraries_override_with_invalid_asset.info.yml
+++ /dev/null
@@ -1,9 +0,0 @@
-name: 'Test theme libraries-override'
-type: theme
-description: 'Theme with invalid libraries-override asset spec.'
-version: VERSION
-base theme: classy
-core: 8.x
-libraries-override:
-  # A malformed library asset name. Should throw a \LogicException.
-  core/drupal.dialog/css: false