Do not try the "rockbox_failsafe" theme!
When I did, the screen went black. The top bar is black text on gray, but the rest is black on black. Fortunately, half of the RB menu is black on gray, and so it's possible to correct this mistake without uninstalling Rockbox, but it isn't fun.
Note: It was on a Sansa C240 v1. I deleted the rockbox_failsafe.cfg file (hopefully that wasn't a bad idea).