It just depends on how detailed the theme is. Some only do icon sets and some redo the entire UI. Read the description and look at the screen shots before purchasing it and it will answer ur questions. Also look in the theme thread to see how it looks on others phones. There are a few people using it. Good luck
It's just how rdiddy_25 explained, It depends on how detailed the theme is.
Some themes are divided in many parts, for example ayecon, so you can select what you want to have themed (see screenshot).
You can also use many different themes together or mix theme parts. That's what I did las year a lot and it's just amazing