DEV: Show warning in admin UI for core SCSS color vars (#12984)

Co-authored-by: Jarek Radosz <jradosz@gmail.com>

app/assets/javascripts/admin/addon/components/admin-theme-editor.js

@@ -6,6 +6,8 @@ import { isDocumentRTL } from "discourse/lib/text-direction";
 import { next } from "@ember/runloop";
 
 export default Component.extend({
+  warning: null,
+
   @discourseComputed("theme.targets", "onlyOverridden", "showAdvanced")
   visibleTargets(targets, onlyOverridden, showAdvanced) {
     return targets.filter((target) => {
@@ -124,5 +126,9 @@ export default Component.extend({
     save() {
       this.attrs.save();
     },
+
+    setWarning(message) {
+      this.set("warning", message);
+    },
   },
 });