I've followed the instructions included. But for some reason my skin isn't showing up in Preferences. Any ideas guys?
To make a skin, you need to take a copy of resource/styles/steam.styles file and copy it to skins/<your skin name here>/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!