Wonderful work! Thanks for sharing this!
BTW I just happened to notice a glitch since I was using menu bar below caption bar.
With skinned borders, switching from "menu bar below caption bar" to "caption bar (button)" leaves menu bar separator drawn over main player docked on the top. Probably the line comes from:
<element id="MenuBarSeparator" bg="208,107,49" />
But I don't know how to fix it, except restarting MB, since I've never worked on bitmap skins.