I was going to post about editing a Rockbox WPS and the struggles I went through to tweak one I liked (did A LOT of reading and experimenting), when I stumbled upon this:
No idea if it's any good, but it's the first I've heard of it.
Another invaluable tool is the Rockbox Simulator, which you can find on the site. DAMN handy.
I'd definitely take the time to read about the elements in a WPS and mess around in the simulator. I was able to tweak one to display more info like Artist | Album | Year on one line, Title and Time on another, both scrolling, as needed. Also managed to tweak the album art display area so the bar with two lines of tag data didn't sit on top of it (just resized the area, basically, though tweaking it to be just so took a lot work). Then I shamelessly saved it under my own name ;-)
EDIT: I used the Rockbox manual (PDF) for learning about editing themes. As when I first learned HTML, I found it useful to work off a copy of a theme I liked. And don't forget the simulator! Saves an amazing amount of time vs. loading and checking on the device!
Edited by ThurstonX - 11/3/12 at 9:18am