What do you mean by themes preinstalled? If you want to change how your phone looks, you have a lot of options without rooting.
1. Use SprintID: these are preconfigured themes you can download and use. Just hit your SprintID icon and follow the directions (personally I've never used it).
2. Go to the Market and download a new live wallpaper. It's amazing how much this can change the look of your phone. Stick to the popular ones; they'll be the least likely to invoke abnormal battery drain.
3. If you're feeling a little more adventurous, install an alternate launcher like ADW, GoLauncher, or Launcher Plus (all free, but most have paid/donation versions as well). These will allow you to change all sorts of things on how your phone looks and behaves. They can be a little intimidating at first, but just play with the options and available themes (there are a ton for all of them). You can have a different look every day of the week if you so desire.
Good luck and have fun!