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/<your skin name here>/resource/styles/steam.styles. 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!
Hitting F6 key when running the Steam client toggles the VGUI editing dialog. You need to run Steam with the -dev command line parameter to have this work (running with -dev makes the Steam client run slower). When enabled, no other dialog can be interacted with; instead you can selected controls on any active dialog and get details on the selection.