Skin themes provide for the ability of skin developers to create variants of existing skins. A skin theme is a collections of skin xmls, image files, etc. that vary from the "base" skin acording to the intent of the theme. Skin themes can be useful for providing, for example, seasonal variations of a skin (e.g., a Christmas theme).
It's a lot of work to create a complete skin including plugins and we appreciate those who work so hard to produce the great skins for MediaPortal. Several skins come in many flavors including new concepts from the ground up as well as slight to major modifications of existing skins. The main objective of skin themes is to make it easier to create new skins or variants of existing skins. You can start with a skin you like and simply make some tweaks to it to change colors, icons, fonts, etc. The beauty of using a theme is that the authors base skin is never changed, none of it, not a bit. This allows the original skin author to make updates to their own base skin while others may contribute to the themes for that same skin. This is just one scenario that we believe can lead to a really great skins each with multiple themes.
When you intermix the use of themes with the skin expressions there is a whole new world of possibilities for MediaPortal.
The use of themes is optional and none of what is explained here is necessary for skins to continue to work without themes - this implementation is designed to be 100% backward compatible.
See also: Skin Themes