Issue #2404253 by wuinfo, mermentau: Undefined index: css in color_library_info_alter()

core/modules/color/color.module

@@ -76,6 +76,7 @@ function color_library_info_alter(&$libraries, $extension) {
     $color_paths = \Drupal::config('color.theme.' . $extension)->get('stylesheets');
     if (!empty($color_paths)) {
       foreach (array_keys($libraries) as $name) {
+        if(isset($libraries[$name]['css'])) {
           // Override stylesheets.
           foreach ($libraries[$name]['css'] as $category => $css_assets) {
             foreach ($css_assets as $path  => $metadata) {
@@ -102,6 +103,7 @@ function color_library_info_alter(&$libraries, $extension) {
         }
       }
     }
+  }
 }
 
 /**

core/modules/color/tests/modules/color_test/themes/color_test_theme/color_test_theme.libraries.yml

@@ -3,3 +3,10 @@ base:
   css:
     theme:
       css/colors.css: {}
+
+fontsizer:
+  version: VERSION
+  js:
+    js/color_test_theme-fontsize.js: {}
+  dependencies:
+    - core/jquery

core/modules/color/tests/modules/color_test/themes/color_test_theme/js/color_test_theme-fontsize.js

@@ -0,0 +1,9 @@
+/**
+ * @file
+ * Adds javascript functions for font resizing.
+ */
+(function ($) {
+  "use strict";
+
+  $(document).ready(function () {});
+})(jQuery);