[RFR] Fix Layout styles can't be overridden by theme

[leonidasv]

Right now, it's not possible to target the Layout component via a custom theme.

This PR aims to fix that, thus making code like the following work:

const theme = createMuiTheme({
  overrides: {
    RaLayout: {
      root: {
        display: 'contents'
      },
      appFrame: {
        marginTop: '5rem'
      }
    }
  }
});

export default function Home() {
  return (
    <Admin theme={theme}>
      {/* your Resources here */}
    </Admin>
  );
}

Otherwise, you would need to create a custom <Layout /> component using withStyles() to merge the changes and pass this custom component to the <Admin /> component.

[fzaninotto]

Thanks!