Instructions
Download a skin for Steam and save it to your computer in the "Steam\skins" folder. Various websites allow users to download skins for free (See "Resources" below).
Close and restart Steam if you are using the program.
Log into your account.
Click "View" and then "Settings" on the top toolbar.
Select the "Interface" tab.
Choose the skin that you downloaded from the drop-down menu under "Skins."
Restart Steam in order to activate the changes.