theme: declare default v5 theme

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/theme/src/v5/types.ts

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+import { Theme } from '@mui/material/styles';
 import {
   BackstagePaletteAdditions,
   BackstageThemeAdditions,
@@ -31,6 +32,10 @@ declare module '@mui/material/styles/createTheme' {
   interface ThemeOptions extends BackstageThemeAdditions {}
 }
 
+declare module '@mui/private-theming/defaultTheme' {
+  interface DefaultTheme extends Theme {}
+}
+
 // This is a workaround for missing methods in React 17 that MUI v5 depends on
 // See https://github.com/mui/material-ui/issues/35287
 declare global {