How do I set the background color of the RootDockingPanel?

[i-make-robots]

I tried

RootDockingPanel root = new RootDockingPanel(this);
add(root, BorderLayout.CENTER);
root.setBackground(Color.DARK_GRAY);

then removed all panels. The background of the root panel is white.

[andrewauclair]

What's the full code? I can't reproduce it with a simple test.