I'm happy to show you a first screenshot of our new Stage style and our MDI system:
Compared to the standard stage:
Sure, the default stage is OK, but if you want to style the whole application, it won't work with standard stage. If you want a unique style, you need a custom solution. Our style is part of our JavaFX UI for JVx and already available in our repository at sourceforge.
The MDI system is already stable. It can be styled via css and works similar to JDesktopPane and JInternalFrame of Swing. Most problems were solved and we use the implementation in our dev projects.