Add changeset

Signed-off-by: Philipp Hugenroth <philipph@spotify.com>

.changeset/weak-snails-sparkle.md

@@ -0,0 +1,24 @@
+---
+'@backstage/app-defaults': minor
+'@backstage/test-utils': minor
+'@backstage/theme': minor
+'@backstage/plugin-techdocs-addons-test-utils': patch
+'@backstage/cli': patch
+---
+
+**MUI v5 Support:** Adding platform-wide support for MUI v5 allowing a transition phase for migrating central plugins & components over. We still support v4 instances & plugins by adding a
+
+To allow the future support of plugins & components using MUI v5 you want to upgrade your `AppTheme`'s to using the `UnifiedThemeProvider`
+
+```diff
+     Provider: ({ children }) => (
+-    <ThemeProvider theme={lightTheme}>
+-      <CssBaseline>{children}</CssBaseline>
+-    </ThemeProvider>
++    <UnifiedThemeProvider theme={builtinThemes.light} children={children} />
+     ),
+```
+
+**'@backstage/test-utils':** Test App Wrapper is using `UnifiedThemeProvider` for tests now.
+**'@backstage/plugin-techdocs-addons-test-utils':** Fix avoiding test to run on unmount.
+**'@backstage/cli':** Enforcing MUI v5 specific linting to minimize bundle size.

packages/theme/src/unified/overrides.test.ts

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import { Theme } from '@mui/material';
+import { Theme } from '@mui/material/styles';
 import { transformV5ComponentThemesToV4 } from './overrides';
 
 describe('transformV5ComponentThemesToV4', () => {