Modena.css specifies the background as -fx-background: derive(-fx-base, 26.4%); which should give us #F4F4F4. If you run the attached code, you'll notice that if you use:
Scene scene = new Scene(new Group(root), 300, 250);

the background of the window is white. On the other hand, if you use e.g.:
Scene scene = new Scene(root, 300, 250);

the background is correctly set (#F4F4F4). It means that Group does not inherit from .root.

Attached are screenshots of the app in Modena normal and white on black theme.