Using a jail broken iPad, you go to Cydia (the jail breaker's "App Store") and download either Winterboard or Dreamboard themes. Some are paid, some are free. Then, after installation, you activate them (with Winterboard, in the Settings app ... Im unsure about Dreamboard) and you're done.
Specifically, you change them (in Winterboard) by putting a check mark next to the theme you want to activate and ensuring that theme is at the top of the list. Then, you respring the iPad and when it's finished with that, your theme is showing.
There are hundreds, if not thousands of themes out there. All it takes is some time in Cydia to search for them.
Hope this clarifies.
Marilyn