You can also set your font, background, and foreground in the config file that frequently is distributed with your wps. If you look at any of my wps's for the apple iPod 5g, they each contain a config file as well as a wps. It's in /.rockbox/themes/ and has a CFG extension.
As an example, here's the cfg file for my theme WLights:
# WLights.cfg (White Lights)
# Made by Wesley Clifford (PlanetRetcon)
wps: /.rockbox/wps/WLights.wps
font: /.rockbox/fonts/nimbus-12.fnt
backdrop: /.rockbox/backdrops/WLights.bmp
statusbar: on
foreground color: 000000
background color: FFFFFF
I'm actually a bit surprised this isn't in the wiki, at least not in the two places I know to look for wps stuff. Granted, it's not in the wps file but it goes hand-in-hand with it.