well.... all black can mean various things... first you should restore the old one to make sure nothing else is weird.then make a new THEME folder, like
WoS\Themes\Darangen\Art
and put a copy of buttonBar.bmp in there.
Then modify that copy and make sure the modded version is the same dimensions and is still a 256 color RLE bitmap (like a skin file).
Shouldn't be anything *too* magical about it.