diff --git a/core/modules/ckeditor5/ckeditor5.libraries.yml b/core/modules/ckeditor5/ckeditor5.libraries.yml
index 61e1d9e63cd6c2dba2de8fd64dce843023dfdf82..774d1f5f8a7e48d626a7b85ae5be2c6dd96d0ae8 100644
--- a/core/modules/ckeditor5/ckeditor5.libraries.yml
+++ b/core/modules/ckeditor5/ckeditor5.libraries.yml
@@ -17,6 +17,9 @@ drupal.ckeditor5.internal:
 drupal.ckeditor5:
   js:
     js/ckeditor5.js: {}
+  css:
+    theme:
+      css/editor.css: { }
   dependencies:
     - core/jquery
     - core/once
diff --git a/core/modules/ckeditor5/css/editor.css b/core/modules/ckeditor5/css/editor.css
new file mode 100644
index 0000000000000000000000000000000000000000..5c28250f972547f51855d7c4150bccd5211cfe0f
--- /dev/null
+++ b/core/modules/ckeditor5/css/editor.css
@@ -0,0 +1,10 @@
+/**
+ * @file
+ * Styles for the CKEditor 5 editor.
+ */
+
+/* Convert low opacity icons to full opacity. */
+.ck-button:not(.ck-disabled) .ck-icon * {
+  opacity: 1 !important;
+  fill-opacity: 1 !important;
+}