If all you want to customize in Firefox is the toolbars background, there is an extension that specializes in just that: AnyColor.
Created by Pavlos Touboulidis, it allows to easily change the background, text and highlights colors, or use a background image for the upper toolbars area and the status bar, set whether to repeat or not them, and its position. You can also save your customization as a preset or use any of the dozen included for quick access.
AnyColor is similar to Personas, a Mozilla Labs extension, in that you can use an image as a background, but the latter lacks the option to set basic colors. On the other hand Personas, allows users to use remote images and even a web page as the toolbars’ background which just expands the possibilities.
AnyColor is available for download at Mozilla Add-ons, but you will need an account there as it is still labeled experimental.