chore: revert all previous changes

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

.changeset/vast-jeans-boil.md

@@ -1,22 +0,0 @@
----
-'@backstage/create-app': minor
-'@backstage/theme': 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/src/index.tsx
-
-+ 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/app-legacy/src/index.tsx

@@ -14,7 +14,6 @@
  * limitations under the License.
  */
 
-import '@backstage/theme/MuiClassNameSetup';
 import '@backstage/cli/asset-types';
 import ReactDOM from 'react-dom/client';
 import App from './App';

packages/app/src/index.tsx

@@ -14,7 +14,6 @@
  * limitations under the License.
  */
 
-import '@backstage/theme/MuiClassNameSetup';
 import '@backstage/cli/asset-types';
 import ReactDOM from 'react-dom/client';
 import app from './App';

packages/create-app/templates/default-app/packages/app/src/index.tsx

@@ -1,4 +1,3 @@
-import '@backstage/theme/MuiClassNameSetup';
 import '@backstage/cli/asset-types';
 import ReactDOM from 'react-dom/client';
 import App from './App';

packages/create-app/templates/next-app/packages/app/src/index.tsx

@@ -1,4 +1,3 @@
-import '@backstage/theme/MuiClassNameSetup';
 import '@backstage/cli/asset-types';
 import ReactDOM from 'react-dom/client';
 import App from './App';

packages/theme/package.json

@@ -6,7 +6,9 @@
     "role": "web-library"
   },
   "publishConfig": {
-    "access": "public"
+    "access": "public",
+    "main": "dist/index.esm.js",
+    "types": "dist/index.d.ts"
   },
   "keywords": [
     "backstage"
@@ -18,25 +20,7 @@
     "directory": "packages/theme"
   },
   "license": "Apache-2.0",
-  "sideEffects": [
-    "./src/unified/MuiClassNameSetup.ts",
-    "./dist/MuiClassNameSetup.*"
-  ],
-  "exports": {
-    ".": "./src/index.ts",
-    "./MuiClassNameSetup": "./src/unified/MuiClassNameSetup.ts",
-    "./package.json": "./package.json"
-  },
-  "typesVersions": {
-    "*": {
-      "MuiClassNameSetup": [
-        "src/unified/MuiClassNameSetup.ts"
-      ],
-      "package.json": [
-        "package.json"
-      ]
-    }
-  },
+  "sideEffects": false,
   "main": "src/index.ts",
   "types": "src/index.ts",
   "files": [

packages/theme/report-MuiClassNameSetup.api.md

@@ -1,7 +0,0 @@
-## API Report File for "@backstage/theme"
-
-> Do not edit this file. It is a report generated by [API Extractor](https://api-extractor.com/).
-
-```ts
-// (No @packageDocumentation comment for this package)
-```

packages/theme/src/unified/MuiClassNameSetup.ts

@@ -1,41 +0,0 @@
-/*
- * Copyright 2026 The Backstage Authors
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-import { unstable_ClassNameGenerator as ClassNameGenerator } from '@mui/material/className';
-
-/**
- * This API is introduced in @mui/material (v5.0.5) as a replacement of deprecated createGenerateClassName & only affects v5 Material UI components from `@mui/*`.
- *
- * 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
- *
- * ```diff
- * // packages/app/src/index.tsx
- *
- * + 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);
- * ```
- */
-ClassNameGenerator.configure(componentName => {
-  return componentName.startsWith('v5-')
-    ? componentName
-    : `v5-${componentName}`;
-});
-
-export {};

packages/theme/src/unified/UnifiedThemeProvider.tsx

@@ -14,7 +14,6 @@
  * limitations under the License.
  */
 
-import './MuiClassNameSetup';
 import { ReactNode } from 'react';
 import {
   ThemeProvider,
@@ -28,6 +27,7 @@ import {
   Theme as Mui5Theme,
 } from '@mui/material/styles';
 import { UnifiedTheme } from './types';
+import { unstable_ClassNameGenerator as ClassNameGenerator } from '@mui/material/className';
 
 /**
  * Props for {@link UnifiedThemeProvider}.
@@ -41,6 +41,15 @@ export interface UnifiedThemeProviderProps {
   themeName?: string;
 }
 
+/**
+ * This API is introduced in @mui/material (v5.0.5) as a replacement of deprecated createGenerateClassName & only affects v5 Material UI components from `@mui/*`.
+ *
+ * This call needs to be in the same module as the `UnifiedThemeProvider` to ensure that it doesn't get removed by tree shaking
+ */
+ClassNameGenerator.configure(componentName => {
+  return `v5-${componentName}`;
+});
+
 // Background at https://mui.com/x/migration/migration-data-grid-v4/#using-mui-core-v4-with-v5
 // Rather than disabling globals and custom seed, we instead only set a production prefix that
 // won't collide with Material UI 5 styles. We've already got the separate class name generator