I have found a few issues similar when customizing controls. When you cover a control with a decoration the decoration is infront of the control, so the control will not have focus to generate events, you can however do two things with a button, place the decoration behind the Button and make the button transparent or you can modify the pictures embedded (4 independent states pictures) associated with a button, or some combo of the two. I still find it hard to get custom controls to look exactly as I intend when I envision something complex, There is probably some art and much creative thought to getting a successful GUI element. Good luck.
-Paul