http://www.rockbox.org/wiki/ThemeGuidelines

http://www.rockbox.org/wiki/CustomWPS

That's how I figured out how to make my theme (I also used original code from cabbiev2, that is by default included in rockbox for n900).
There's no Theme Editor "GUI" program working properly, so the best way for you to create a theme is by using a ssh connection and then use WinSCP (or whatever alternative for your OS) to edit .wps, .sbs and .cfg files wirelessly without cable hassle. (thus preventing it from breaking...)