You should be able to set the colors to anything you like with a bit of manual file editing.
The basic Mozilla preferences are in a file called prefs.js. Rather then edit this file manually, you should create a file called user.js in the same directory. Then copy any lines from prefs.js you want to change into user.js and edit them there.
The advantage of user.js is that Mozilla will read this file but doesn't change it, so you won't have problems with Mozilla undoing or altering changes.
You should be looking for a line that looks something like this.
user_pref("browser.display.background_color", "#AAAAAA");
If you can't find what you are looking for, go into Mozilla and change the value to something other then the default and then exit Mozilla. This will force Mozilla to put the value in the prefs.js if it wasn't already there.
[link|http://www.mozilla.org/unix/customizing.html|Customzing Mozilla] covers this in more detail.
Jay