There are many wallpaper apps in the App Store (and there are a whole lot of pictures available on the Internet) for you to select a new background picture on the iPad. Then, there are various apps in the App Store that will let you set up different backgrounds to make it look as if you have shelves and such. One such app is Pimp Your Screen ($0.99, USD). It's really neat and I am sure there are more like that.
For more intense, detailed theming, you have to be jailbroken to add themes. And you get them from Cydia, the app "store" for jailbreakers. If you are not jailbroke, you don't get access to Cydia.
So, either go with changing wallpapers or jail break your iPad for more intense themes. Good luck.
Marilyn