GuiWindow - Make Transparent

I have no problem making the borders and title bar of a window control transparent. My profile is:
, but can't get the body of the window to go transparent.

new GuiControlProfile (GuiWindowProfile)
{
opaque = false;
border = true;
fillColor = "128 128 128";
fillColorHL = "128 128 128";
fillColorNA = "128 128 128";
fontColor = "255 255 255";
fontColorHL = "255 255 255";
text = "GuiWindowCtrl test";
bitmap = "./demoWindow";
textOffset = "6 6";
hasBitmapArray = true;
justify = "center";
};

No matter what I set the fillColor to, even with opaque set to false, I can't get a the body of the window to go transparent for me.

Does anyone have any inside tips on how to make this happen? Thanks!

-Robert