STEP 1: You need a jailbroken device to perform this guide, you can use our guide posted HERE to jailbreak your iOS 4.2.1 device using Redsn0w.
STEP 2: Open Cydia, tap on Manage, then Source, edit and add the below repository source as shown in the screenshot below:
STEP 3: Return back, navigate to the source and install Theme It app as shown below:
STEP 4: That's it, now you'll get Theme It on your home screen, launch it and have fun !
LINK ; REDSNOW