message: Added example of icon for theme
diff: @@ -57,6 +57,7 @@ done like this:
  import { createApp } from '@backstage/app-defaults';
  import { ThemeProvider } from '@material-ui/core/styles';
  import CssBaseline from '@material-ui/core/CssBaseline';
  +import LightIcon from '@material-ui/icons/WbSunny';
  const app = createApp({
  apis: ...,
  @@ -65,6 +66,7 @@ const app = createApp({
  id: 'my-theme',
  title: 'My Custom Theme',
  variant: 'light',
  + icon: <LightIcon />,
  Provider: ({ children }) => (
  <ThemeProvider theme={myTheme}>
  <CssBaseline>{children}</CssBaseline>
  