chore: add changeset

Signed-off-by: Rajib Quayum <rajibq@users.noreply.github.com>

.changeset/vast-jeans-boil.md

@@ -0,0 +1,24 @@
+---
+'example-app-legacy': minor
+'@backstage/create-app': minor
+'@backstage/theme': minor
+'example-app': minor
+---
+
+Separates MUI 5 class name generator code into separate file and entry point that can be imported before any MUI 5 component loads.
+
+This addresses the problem where the elements contain the `v5-` prefix for MUI class names, but the static class names from the library do not.
+
+Import should be made as follows to ensure the prefix problem is addressed correctly:
+
+```diff
+// packages/app/index.ts
+
++ import '@backstage/theme/MuiClassNameSetup'; // must be the very first import!
+import '@backstage/cli/asset-types';
+import ReactDOM from 'react-dom/client';
+import app from './App';
+import '@backstage/ui/css/styles.css';
+
+ReactDOM.createRoot(document.getElementById('root')!).render(app);
+```

packages/theme/src/unified/MuiClassNameSetup.ts

@@ -20,9 +20,10 @@ import { unstable_ClassNameGenerator as ClassNameGenerator } from '@mui/material
  *
  * This needs to be configured before any MUI 5 component can possibly be imported. See: https://v5.mui.com/material-ui/experimental-api/classname-generator/#caveat
  *
- * ```packages/app/index.ts
+ * ```diff
+ * // packages/app/index.ts
  *
- * import '@backstage/theme/MuiClassNameSetup'; // must be the very first import!
+ * + import '@backstage/theme/MuiClassNameSetup'; // must be the very first import!
  * import '@backstage/cli/asset-types';
  * import ReactDOM from 'react-dom/client';
  * import app from './App';