I've been building a basic theme. I've got "Composition=0" in my theme file so it shows up as a basic theme and loads the relevant parts etc just fine. But now I delete that line, it still just loads the basic theme resources in the .msstyles file. Microsoft theme files are able to switch between aero and basic from the same aero.msstyles file.

I don't know what's happened to break it. I am going to have to rebuild the theme from the beginning if I can't fix it Any ideas?