message: Improve documentation
  `defaultPageTheme` is a required field.
diff: @@ -27,6 +27,7 @@ import { createTheme, lightTheme } from '@backstage/theme';
  const myTheme = createTheme({
  palette: lightTheme.palette,
  fontFamily: 'Comic Sans MS',
  + defaultPageTheme: 'home',
  });
  ```
  