Use Winterboard, it's available in both Installer and Cydia.
If you have Cydia:
Open, select Search and type in Winterboard. Select it and install. Now go back to your homescreen. There will now be an app called, obviously, Winterboard. However, there won't be any themes in it.
Reopen Cydia, and download some themes (if you search on the net and find a theme you really like, try and search for it in Cydia and download it). After Cydia 'installs' the theme, go to the homescreen and open Winterboard. Select the theme you just downloaded and exit. As Winterboard closes, Springboard (basically the background process that controls your home screen) will restart and you will see the 'Slide to Unlock' screen after the theme is applied. Slide to unlock and your home screen should now be themed.