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.
While in any Steam window you if you set up the
-dev launch option correctly you should be able to press 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.
More information for creating/editing skins can be found on the VGUI Editing page.
steamskins.org, a website with tens of skins for Steam
steamcustomizer.com, an easy to use skin generator