Tabbed button colour, Win XP

[Martin BLACKMAN]

I have a tabbed button in Rev 2.6 whose background color is set to dark grey.  
In XP with the theme set to 'windows classic' it appears as dark grey.
With the theme set to 'Windows XP' the background of the button
becomes off-white, and setting the background color of the button has
no effect.

I use this button effectively as the background for most of my card,
and switch between cards using the tabs.

Can someone suggest a way to force the colour to grey ?  I could put a
grey rectangle on top but that would still leave the tabs...

thanks
Martin Blackman