I figured it out. It looks like importing the CSS does the trick:

import '@medplum/react/styles.css';
import '@medplum/react/defaulttheme.css';

Glad that worked! We're working on improving our documentation so that it is less confusing going forward. Would you mind telling me which documentation pages you were looking at for instruction? I'll update htem

This: https://www.medplum.com/docs/tutorials/ui-components/hello-world-part-1

Great! We'll get that updated