Here's a few example images.
Left: black button corners and 10% transparent window background (no alpha channel in the tga)
Middle: transparent corners, game graphics shining through
Right: Using Overlay blend mode, corners blend with window background but the underlying quickslot shines through (yeah it's a bit complicated heh)
The images are 300% zoomed so left and right images don't look that bad in-game.
-Chiran