JFrame has a frameInit method that does the following:

setBackground(UIManager.getColor("control"));

This sets the background based on the current L&F. However, the background isn't changed on a L&F update.

This isn't a major problem, since the color is only shown when resizing (and only when dynamic layout is off). However, we need to investigate making it do the right thing. This includes looking at all Swing top levels.