Skinning refers broadly to the ability to customize the user interface. The Skinning project is part of the User Interface Roadmap and specifically describes a path to make the Second Life user interface entirely data driven and customizable by Linden Lab, 3rd party developers, and individuals, without requiring changes to the primary application source code, and maintainable across updates. Learn more below!
http://wiki.secondlife.com/wiki/Skinning
http://wiki.secondlife.com/wiki/Category:Viewer_Skin_Images
http://wiki.secondlife.com/wiki/Category:Viewer_Skins
http://wiki.secondlife.com/wiki/Skinning_HowTo/Basics
http://wiki.secondlife.com/wiki/Skinning_HowTo/Increase_the_transparency_of_unfocused_Chat_windows
Daniel Voyager 