I find that color annoying as well. I hope that FF does A LOT of tweaking before they release the final full version of 4.0. In my opinion they are trying to "look" too much like Chrome and that's a big mistake as far as I'm concerned.
You "might" be able to change look through Windows Aero Themes but I don't know that for a fact.
Here's a link to the latest 3.6 version which should take you back to the look you want. Install version 3.6.16 and then uninstall 4.0 to get your disc space back.
Fuagf: the color you are talking about is a "theme" and it used to be a button at the top of the add-on row, in between the add on and the extension buttons.
I cannot open 4 since it wipes out many settings in 3.6! But when I was playing with it in the beta, was able to get it back to the original default.
Firefox now has personas that you can use to change your backgrounds. Just hover over any of them to see what it looks like and then click on it to use it.