How do I make a toolbar with transparent icons

[kmbruhnk rockwellcollins com]

Hello,

I need to make transparent toolbars and panels that can fade after not 
being interacted with for a certain period of time. The fade level and 
speed must be user configurable and must apply only to the current 
application, not to the whole desktop environment. These toolbars and 
panels must overlay a viewport which is drawn to by a 3rd-party library.

I'm using a theme that imitates Motif with CDE which I created using the 
pixmap engine. I know its slow, but it works well for this application so 
far. Also, I'm using GTK+ 2 with libglade, but am willing to do this part 
without glade if it will work. I have set RENDER and DAMAGE to true and 
Composite to Enable in my xorg.conf, and I do have an rgba colormap, so 
that part is working.

I thought I could use transparent button images in the theme and use cairo 
to simply set the alpha level to make them fade. Although I can fade the 
toolbar, the underlying viewport does not show through. I simply get a 
black background in the area that I want to be transparent. Now I'm 
thinking I may need an alphachannel-aware theme engine to draw my widgets. 
Murrine might work if I could set the alpha level programmatically, 
instead of setting it in a .h file.

Is there a different theme engine that would be better for my application 
that would allow me to set the alpha level dynamically? Or is there 
another, perhaps better, way to superimpose my toolbar on the viewport? Am 
I just missing something obvious?

Thanks in advance,
Kurt Bruhnke