I have a copy of the Droid Root Helper Pro and it has a couple of options for keyboard mods that make the keyboard red and green respectively. I'm curious as to how one might create and install a mod like that.
You need to edit the .png files in
/system/framework/framework-res.apk (/res/drawable) for the button backgrounds
/system/app/LatinIME (/res/drawable) for the highlights and overlays
Then rebuild the apk and push to the phone, or sign and flash them.