The name says it all: remove the background image and turn the background black. Choose another color if you don't want it to be black. It also puts a hover color (outline) around the 'site settings' link, currently lime, but everything goes with black :D.

Or, as the image below shows, you can use your 'fav' background image (change 'none' to 'url("your.img.url")'). The sample image below is from InterfaceLIFT (http://interfacelift.com/wallpaper_beta/details/1156/darkness_to_light.html) by shiftedreality (http://interfacelift.com/user/6458/shiftedreality.html), but, please, don't point directly to InterfaceLIFT--point to a local file or a file server setup for it (which, like me, you might be running locally). (And, FWIW, those are not my 'fav4' sites--just three of the defaults plus the first one having been changed for "effect".)

FYI: They've added a Themes option, one of which being "Lights out" (so you don't really need this style anymore to get a black background).

Updates...
25Aug2010: removed outline/added transparency for settings img (bottom right of page), hide copyright msg (bottom left)
28May2010: added 'domain(myfav.es)' (due to site "rename" to MyFaves)