message: packages/theme: use Overrides type from MUI
diff: 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 {
  