I hope this gets into the next mainstreme release. Sawmill and Gnome should be so tightly integrated that you download one theme-package, that applies both the GTK and sawmill themes, making them seem one. The ability to mix and match should of course be there, only not selected as default. The higher theme requirements are, the more difficult it will be for anyone to contribute. Most people just prefer a different combinaison than the themes authors.
Integrated themes make for big downloads.
See wprefs if you don't know what a good configuration utility is. Problem is, no one is able to make this kind of package single-handledly. Choosing this path is only opening the way to shitloads of crap. People making the themes could include all of these or just the ones they want to do. Or you could download each one individually.