If you're using the Premier edition, and are talking about the Basic MSI themes, there is no documentation on the format, but it shouldn't be hard to follow and edit a copy of a .theme file (it's XML) or its related imagery. If instead you're talking about InstallScript skins, you'll need the Skin Customization Kit.
A theme file is a . ini text file that is divided into sections, which specify visual elements that appear on a Windows desktop. I've been surfing into themes and templates and just find out there are numerous themes and templates that came handy for my project. Like, there are certain PHP eCommerce Templates that have a list of things and features like:
