I use blackbox and get Pitkon themes from DeviantArt. I can't figure out how you can theme fvwm?
not that the default isn't excellent on it's own. I like the default for switching screens, and the pager is nice, I can change the border of apps, and set the background with bgsetbg.
In blackbox I use the skins from Pitkon's older work, the backgrounds from his newest work, and throw out anything in between that relies on windows.
http://pitkon.deviantart.com/