OK, I didn't solve the problem, but I found a workaround.
To change the WPS (While Playing Screen) or Base Skin files, just edit the \.rockbox\config.cfg in a text browser (notepad, etc). No locate the following lines:
wps: /.rockbox/wps/Hi-Fi.wps
sbs: /.rockbox/wps/Hi-Fi.sbs
And change the lines to whatever you want. You can use the built-in cabbiev2 theme, or you can get a new one from the Rockbox site.
I tried a bunch of the Sansa Clip+ themes (
http://themes.rockbox.org/index.php?target=sansaclipplus), and was able to find some really nice Base Skins and WPSs. I'm sooo glad I did. The ones I'm using now are cleaner, easier to read, cooler looking, etc than the 2 built-in ones. There's really simple themes, nerd overload themes, large text themes, alternate language themes, and even 'themed' themes (Pac-Man, Pokemon, Star Trek, Aliens, etc).
Check them out on that link - there's probably 75 different ones. They're really easy to change (even with themes being broken at the moment). You simply download the .zip files, and extract the files to the .rockbox folder. The new themes will be put in the .rockbox\themes folder, and the new WPS will be put into the .rockbox\wps folder.
Then you just edit the \.rockbox\config.cfg in a text browser (notepad, vi, etc) and change the appropriate lines to what you want.
After trying 15 of them, I narrowed it down to 2 in particular - Sweet and Balanced
So for example, to use Sweet you would change the lines
wps: /.rockbox/wps/Hi-Fi.wps
sbs: /.rockbox/wps/Hi-Fi.sbs
to
wps: /.rockbox/wps/Sweet.wps
sbs: /.rockbox/wps/Sweet.sbs
and save the config.cfg file.
3 things to be careful on is:
1.) Make sure the theme says it "works with current build", and ignore the "Doesn't work with current build" ones.
2.) Make sure the you type the name in the config.cfg file with the same capitalization as the physical file name in the .rockbox\wps folder (so if the physical file is .rockbox\wps\BillyBob.wps, don't type billybob.wps).
3.) Ensure there's a matching file in the .rockbox\wps folder before typing it into the config.cfg file. Not every theme from the website has both a base skin AND a WPS screen! Most are full Themes (with both .sbs Base Skins and .wps WPS screen), but a few are WPS screens ONLY (ie only .wps file with no .sbs file). Rockbox will crash if you add BillyBob.sbs to the config.cfg file if there is no .rockbox\wps\BillyBob.sbs (duh).
4. Make sure the file name on the line that starts with "wps:" line ends in blahblah.wps and the line that starts with "sbs:" line ends in blahblah.sbs. You don't want to mix them up by putting blahblah.wps in the sbs: line. Make sense?
Regarding #3, you can still use WPS-only themes. I'm mixing and matching different Base skins and WPS (Sweet for the base skin and Balanced for the WPS screen because I didn't care for the Balanced base skin but I did like its WPS screen). So my config.cfg file looks like this:
wps: /.rockbox/wps/Balanced.wps
sbs: /.rockbox/wps/Sweet.sbs
It's really not as confusing as it may sound. It takes less than 5 minutes to download a theme, unzip the file, and edit the config.cfg file. It's pretty hard to mess up (and even if you do, you can always just put the original settings back ie Hi-Fi.wps and Hi-Fi.sbs).
Once the themes are fixed (they used to work), you won't have to do the file editing business. You would just download & unzip the files, and use the "Settings - Theme Settings - Browse Theme Files" option in Rockbox to change between them. Rockbox will do the file editing magic for you.
Let me know if you have any questions.