Yea a full theme will switch out everything and usually come in update zip formats
Metamorphs are more for individual things but you can do a full theme with a metamorph as well
You can get themes for stock roms although they're a bit harder to come by but if you find a custom stock rom it's easier or if you just have a custom rom you can find all sorts of themes and what not for it
As for where to get themes if you look in any roms section on the forum i'm sure you'll find a bunch of themes or even if you type in on google like droid 2 themes you'll find something adn teh description of the theme will tell you what rom it's for sometimes you'll find ones for stock but like i said they're a bit harder to find usually just for certain rom versions