mostly in bits and pieces on the forums... although there may have been something added to the docs pages recently... also, the comments in the toolbars.cfg, etc. files give the basic instructions. nothing is too critical as far as sizes, really... you'll want the vertical dimension of your throbber to be a few pixels more than the size of your toolbar buttons, or it'll have space above/below it... pure magenta (255, 0, 255) is transparent...also, there's a resources list that hasn't really taken off yet, but you could try posting to:
http://www.geocrawler.com/lists/3/SourceForge/18726/0/
any specific questions you could also post to this thread, and somebody will probably be able to help out... try a search of the forums first, though.