Steam Skins
Setting Up
The first thing you need to do is make sure steam is not running, then navigate to your steam directory "C:\Program Files(x86)\Steam" and copy the resource folder and paste it into your skin folder in "Steam\skins\Your Skin Name" folder. The resource folder contains all of the images and the assets needed to customize the Steam UI. Then go to your steam shortcut and go to it's properties and select the shortcut tab. In the target box type -dev after the directory. Now when you launch steam go to the settings and select the Interface tab, you can now select your skin.
Changing Colors
While in any Steam window you if you set up the -dev launch option correctly you should be able to press F6 and the "vgui layout debugger" should appear. While the debugger is open you cannot interact with anything in the Steam Client until you close the window. Using the debugger you can (like inspect element in web browsers) see what code and scripts are behind what you click and the colors and you can use it to identify which colors you can edit, and which file houses the code for text and images.
Further Documentation
More information for creating/editing skins can be found on the VGUI Editing page.
Current Support
The current Steam Client Beta has been revamped to use Chromium Embedded Framework. It does not have an option to change skins in the Settings window, and the VGUI Layout Debugger does not appear. While the current version is still in beta, the Skins feature has most likely been removed entirely.
History
Removal of skin support for the Friends List on July 24th 2018
Removal of skin support for the Library on February 1st 2023
Removal of skin support for the main window frame and all additional windows in Steam Beta on May 12th 2023
External Links
steamskins.org, a website with tens of skins for Steam
steamcustomizer.com, an easy to use skin generator