The trick to making themes people will actually use (if that is something that is desirable, that is) is to make them good looking without making them interfere with using the device. That's why Apple's Aqua interface is genius - it's beautiful AND it is extremely usable and doesn't waste any space on the screen on useless fribble like thick borders etc that a vast majority of theme designers for the PC slap on their "artistic" efforts.

I haven't actually tried the editor for the 770 and I doubt if you can do much beyond fiddling with colors (I may be wrong) but making a good looking and very usable "skin" for Windows for instance is darn difficult; I could count the good skins I've seen on the net on the fingers of one hand, or something like that.