You need to replace system files and find a theme to your liking on a website such as http://www.Deviantart.com
where users upload their creations and themes.
The themes usually tell you the special method to install and replace system files. Head on over there and have a look.
If you want to jump straight in and get your feet wet, check out Sico theme and Soft7 theme. They are popular. They may not be what you want, but you'll be on the right track. You'll then know how to do what you want to do.