Yeah the Any Color extension allows you to change the back ground color without changing themes (note that all my icons are the default FF3). It also allows you to use a custom picture (either via a link on the internet, or from your harddrive).
So I did a printscreen, crop, and copy paste till I got an image that was the same color as the theme I was using. Booya
It's really easy, and doesn't require any computer knowledge (unlike this userchrome jazz)