gtk_widget_modify_bg and transparency

[Lars Clausen <lc statsbiblioteket dk>]

So I've read Havoc's mini-FAQ on setting colors for widgets, but I've
had little success with using it on an image-based togglebutton.  I'm
not trying to set a particular color for the background, I just want it
to be transparent, since the image takes care of showing the state. 
gtk_widget_modify_bg() and gtk_style both take colors, no way to set
transparency.  Will I have to make my own button and handle drawing on
my own to do this?

Thanks,
-Lars