svg inside of transparent xul window

Hi,
while working on my svgeditor I found that the svg I put inside of a xul 
stack
becomes invisible when I set the window background to transparent which 
I would like
to do to use a custom titlebar with the ususal round corners...
tried also an xul iframe as container for the div but the iframe is 
rendered completely transparent
and I did not find a way to set the iframe background to solid inside of 
the transparent window...

any ideas out there?

Cordially,
Axel