diff --git a/core/modules/system/src/Tests/Asset/LibraryCompletenessTest.php b/core/modules/system/src/Tests/Asset/LibraryCompletenessTest.php
index ab29e85..48a879b 100644
--- a/core/modules/system/src/Tests/Asset/LibraryCompletenessTest.php
+++ b/core/modules/system/src/Tests/Asset/LibraryCompletenessTest.php
@@ -17,6 +17,27 @@
 class LibraryCompletenessTest extends KernelTestBase {
 
   /**
+   * The theme handler.
+   *
+   * @var \Drupal\Core\Extension\ThemeHandlerInterface
+   */
+  protected $themeHandler;
+
+  /**
+   * The theme initialization.
+   *
+   * @var \Drupal\Core\Theme\ThemeInitializationInterface
+   */
+  protected $themeInitialization;
+
+  /**
+   * The theme manager.
+   *
+   * @var \Drupal\Core\Theme\ThemeManagerInterface
+   */
+  protected $themeManager;
+
+  /**
    * The library discovery service.
    *
    * @var \Drupal\Core\Asset\LibraryDiscoveryInterface
@@ -31,13 +52,42 @@ class LibraryCompletenessTest extends KernelTestBase {
   protected $allModules;
 
   /**
+   * A list of all core themes.
+   *
+   * We hardcode this because test themes don't use a 'package' or 'hidden' key
+   * so we don't have a good way of filtering to only get "real" themes.
+   *
+   * @var string[]
+   */
+  protected $allThemes = [
+    'bartik',
+    'classy',
+    'seven',
+    'stable',
+    'stark',
+  ];
+
+  /**
+   * A list of all paths that have been checked.
+   *
+   * @var array[]
+   */
+  protected $pathsChecked;
+
+  /**
    * {@inheritdoc}
    */
   public function setUp() {
     parent::setUp();
 
+    $this->themeHandler = $this->container->get('theme_handler');
+    $this->themeInitialization = $this->container->get('theme.initialization');
+    $this->themeManager = $this->container->get('theme.manager');
     $this->libraryDiscovery = $this->container->get('library.discovery');
 
+    // Install all core themes.
+    $this->container->get('theme_installer')->install($this->allThemes);
+
     // Enable all core modules.
     $all_modules = system_rebuild_module_data();
     $all_modules = array_filter($all_modules, function ($module) {
@@ -49,19 +99,38 @@ public function setUp() {
       return TRUE;
     });
     $this->allModules = array_keys($all_modules);
-    asort($this->allModules);
+    sort($this->allModules);
     $this->enableModules($this->allModules);
   }
 
   /**
-   * Ensures that all core library files exist.
+   * Ensures that all core module and theme library files exist.
    */
   public function testCoreLibraryCompleteness() {
-    $all_libraries['core'] = $this->libraryDiscovery->getLibrariesByExtension('core');
-    $all_libraries += $this->getAllLibraries();
+    // First verify all libraries with no active theme.
+    $this->verifyLibraryFilesExist($this->getAllLibraries());
+
+    // Then verify all libraries for each core theme. This may seem like
+    // overkill but themes can override and extend other extension's libraries
+    // and these changes are only applied for the active theme.
+    foreach ($this->allThemes as $theme) {
+      $this->themeManager->setActiveTheme($this->themeInitialization->getActiveThemeByName($theme));
+      $this->libraryDiscovery->clearCachedDefinitions();
 
+      $this->verifyLibraryFilesExist($this->getAllLibraries());
+    }
+  }
+
+  /**
+   * Checks that all the library files exist.
+   *
+   * @param array[]
+   *   An array of library definitions, keyed by extension, then by library, and
+   *   so on.
+   */
+  protected function verifyLibraryFilesExist($library_definitions) {
     $root = \Drupal::root();
-    foreach ($all_libraries as $extension => $libraries) {
+    foreach ($library_definitions as $extension => $libraries) {
       foreach ($libraries as $library_name => $library) {
         $libraries_to_skip = [
           // Locale has a "dummy" library that does not actually exist.
@@ -75,7 +144,12 @@ public function testCoreLibraryCompleteness() {
         foreach (['css', 'js'] as $asset_type) {
           foreach ($library[$asset_type] as $asset) {
             $file = $asset['data'];
-            $this->assertTrue(is_file($root . '/' . $file), "$file file referenced from the $extension/$library_name library exists.");
+            $path = $root . '/' . $file;
+            // Only check and assert each file path once.
+            if (!isset($this->pathsChecked[$path])) {
+              $this->assertTrue(is_file($path), "$file file referenced from the $extension/$library_name library exists.");
+              $this->pathsChecked[$path] = TRUE;
+            }
           }
         }
       }
@@ -83,21 +157,34 @@ public function testCoreLibraryCompleteness() {
   }
 
   /**
-   * Gets all libraries for all installed modules.
+   * Gets all libraries for core and all installed modules.
    *
    * @return \Drupal\Core\Extension\Extension[]
    */
   protected function getAllLibraries() {
     $modules = \Drupal::moduleHandler()->getModuleList();
+    $extensions = $modules;
     $module_list = array_keys($modules);
-    asort($module_list);
+    sort($module_list);
     $this->assertEqual($this->allModules, $module_list, 'All core modules are installed.');
 
+    $themes = $this->themeHandler->listInfo();
+    $extensions += $themes;
+    $theme_list = array_keys($themes);
+    sort($theme_list);
+    sort($this->allThemes);
+    $this->assertEqual($this->allThemes, $theme_list, 'All core themes are installed.');
+
+    $libraries['core'] = $this->libraryDiscovery->getLibrariesByExtension('core');
+
     $root = \Drupal::root();
-    foreach ($modules as $module_name => $module) {
-      $library_file = $module->getPath() . '/' . $module_name . '.libraries.yml';
+    // By merging $modules and $themes into $extensions we're assuming that a
+    // module and theme don't share names. If they did, their libraries would
+    // conflict anyway.
+    foreach ($extensions as $extension_name => $extension) {
+      $library_file = $extension->getPath() . '/' . $extension_name . '.libraries.yml';
       if (is_file($root . '/' . $library_file)) {
-        $libraries[$module_name] = $this->libraryDiscovery->getLibrariesByExtension($module_name);
+        $libraries[$extension_name] = $this->libraryDiscovery->getLibrariesByExtension($extension_name);
       }
     }
     return $libraries;