feat(ui_patterns_library): Support multitheme sites

modules/ui_patterns_library/src/Plugin/Deriver/LibraryDeriver.php

@@ -160,21 +160,9 @@ public function getPatterns() {
    * @return array
    *   An array containing directory paths keyed by their extension name.
    */
-  protected function getDirectories() {
-    $default_theme = $this->themeHandler->getDefault();
-    $base_themes = $this->themeHandler->getBaseThemes($this->themeHandler->listInfo(), $default_theme);
-    $theme_directories = $this->themeHandler->getThemeDirectories();
-
-    $directories = [];
-    if (isset($theme_directories[$default_theme])) {
-      $directories[$default_theme] = $theme_directories[$default_theme];
-      foreach ($base_themes as $name => $theme) {
-        $directories[$name] = $theme_directories[$name];
-      }
-    }
-
-    return $directories + $this->moduleHandler->getModuleDirectories();
-  }
+   protected function getDirectories() {
+     return $this->moduleHandler->getModuleDirectories() + $this->themeHandler->getThemeDirectories();
+   }
 
   /**
    * Get extension name that hosts the given YAML definition file.

modules/ui_patterns_library/tests/src/FunctionalJavascript/UiPatternsLibraryAlternativeThemeTest.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace Drupal\Tests\ui_patterns_library\FunctionalJavascript;
+
+/**
+ * Test patterns overview page displays for non-default themes.
+ *
+ * @group ui_patterns_library
+ */
+class UiPatternsLibraryAlternativeThemeTest extends UiPatternsLibraryOverviewTest {
+
+  /**
+   * Default theme.
+   *
+   * @var string
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $active_theme = $this->container->get('theme.initialization')->initTheme('ui_patterns_library_theme_test');
+    $this->container->get('theme.manager')->setActiveTheme($active_theme);
+  }
+
+}