This is a primer on how to create skins for Steam. The information below is extracted from the Valve internal document on how to edit, so it may in some cases be wrong or reference tools that don't exist.
To make a skin, you need to take a copy of resource/styles/steam.styles file and copy it to skins//resource/styles/steam.styes. The existance of that file will make Steam put that skin as an option in the settings->interface dialog (Steam will need to be restarted for it to show). From there you can start editing.
You can put new files or existing steam files you want to replace under your skins folder. Good luck!