Merge pull request #21687 from backstage/philipph/mui5-dark-paper

Fix MuiPaper background color in v5 default dark mode

.changeset/dirty-turkeys-reflect.md

@@ -0,0 +1,5 @@
+---
+'@backstage/theme': patch
+---
+
+Align Material UI v5 `Paper` component background color in dark mode to v4.

packages/theme/src/v5/defaultComponentThemes.ts

@@ -239,4 +239,7 @@ export const defaultComponentThemes: ThemeOptions['components'] = {
       underline: 'hover',
     },
   },
+  MuiPaper: {
+    styleOverrides: { root: { backgroundImage: 'unset' } },
+  },
 };