1. CM7 and MIUI are both very stable and have more features working than stock.
2. If you just want to change icons, you can use a 3rd party launcher. They have options to change icons (I know adw and golauncher both do, I am sure others do as well).
3. CM7 and MIUI both have lots of theming options, but yo can do most of that on stock as well, you just have to get in the files more.
There are plenty of theming threads at various places (xda, etc.) just search for some and read up. You can customize any part of the OS if you know what you are doing. Just make sure you search for threads for your version of android (froyo 2.2.2,CM7, MIUI, etc). There are also a decent number of rethemed stock ROMs in the forums here.