Today a friend and I ran into this tool by chance, we decided to try it out, and it turned out to work just fine. It is a lightweight web based editor that allows you to go in and change the color of most things in the steam UI, you can even use custom pictures with your stuff. After finishing it you can download the file and use their manager program to install the theme.

This is to the main page:
http://steamcustomizer.com/

This is my gallery so far:
http://community.steamcustomizer.com/id/ColbyFox

A couple of tips I've noticed from the program so far. You can use multiple themes by renaming the folder of the skin from "Enhanced Steam" to anything you like, this also makes it so installing a new theme does not replace others you like. Also using larger resolution images for customs and shrinking them to the size you need looks better strangely rather than having a smaller image that fits as is. I don't understand it, but I suppose I will not question it much either.