Ah. I just answered your other post about the wallpaper - let me take a stab at your theme question...
If you have a theme, I am going to assume you have the tweak Winterboard (that you got free from Cydia)? So, go to your Settings app and scroll down the left side until you see the line titled "Winterboard." After you press that Winterboard line, you should see a line on the right side titled "Select Themes." Pressing that will bring you to a new page.
Your theme should be listed there. Press the line it's on to get a check mark (which means that you've activated it). Now, up at the top left side, press the button titled "Winterboard." You'll go back to that first screen and you should see another button on the top left titled "Respring." Press that button and your iPad should respring. When it does, and you see all your icons again, that theme should be active and working on your iPad.
Hope that helps. Please, let us know how you get on.
Marilyn