Reuse colors from Frame with Form on BoxPane with Form

[Edvin Syse]

My loginscreen is a Form living in a Frame. This gives a nice gray 
background with white form fields. When I create a Form in a BoxPane, I 
get a light background and the form fields seems to inherit the same 
background instead of being white.

What would be the best way to reuse the colors from the Frame in my BoxPane?
-- Edvin