Accordion background transparent

This really drives me crazy. Is it possible to change the background alpha of an accordion?
The default color is white and alpha 1. I’d like to have a transparent background instead of that white. I am using flex 4/Flash Builder (Gumbo)
The backgroundAlpha property is not working (link:they said it should work)

<mx:Accordion width="100%" height="100%" backgroundAlpha="0.0" />

any idea?