message: Add pointer to custom theme on demo site
diff: @@ -149,6 +149,11 @@ const myTheme = createTheme({
  });
  ```
  +For a more complete example of a custom theme including Backstage and
  +Material-UI component overrides, see the [Aperture
  +theme](https://github.com/backstage/demo/blob/master/packages/app/src/theme/aperture.ts)
  +from the [Backstage demo site](https://demo.backstage.io).
  +
  ## Overriding Backstage and Material UI components styles
  When creating a custom theme you would be applying different values to
  