packages/theme: use Overrides type from MUI

packages/theme/src/baseTheme.ts

@@ -16,6 +16,7 @@
 
 import { createMuiTheme } from '@material-ui/core';
 import { darken, lighten } from '@material-ui/core/styles/colorManipulator';
+import { Overrides } from '@material-ui/core/styles/overrides';
 
 import {
   BackstageTheme,
@@ -23,8 +24,6 @@ import {
   BackstagePaletteOptions,
 } from './types';
 
-type Overrides = Partial<BackstageTheme['overrides']>;
-
 export function createThemeOptions(
   palette: BackstagePaletteOptions,
 ): BackstageThemeOptions {