You can optionally name containers using a @container/{name} class, and then include that name in the container variants using classes like @lg/{name}:underline: @xs @container (min-width: 20rem /* ...
If you don't use Java modules in your application, you also have to import the javafx.controls and javafx.fxml modules: Then, take a look at the JavadocViewerApp class of javadocviewer-app to see an ...