A more detailed explanation for the teckie ones
SMF comes with a Theme called "SMF Core Default" (or something like that). When the forum doesn't find something it's looking for in a customer theme, it goes back to check the default one. That allows you to create new themes by only coying/editing the files that actually need to be changed.
SMF also has an option to choose which theme will be the default in the forum and to disallow the selection of "SMF Core Default" by users. This way, you never have to modify the original theme so you don't get screwed ending up having to redo all your changes whenver an update to the forum replaces a file.
The back side is that mod installation don't take other themes into account, they only modify the "SMF Core Default" so if a mod changed anything in the display templates, I have to find those changes and add them in my four Themes.
Fortunately, the next version of SMF (which doesn't have a release date yet) will fix that and installing mods will update all the themes installed.
