If you want a decent quality theme you'll want to create a 3 second loop with 34 '1280x720' images. It's a bit of a process though. You'll want to convert your video to MOV or AVI. Import it to Photoshop as frames to layers. There should be an option to open every other frame. Isolate the 34 frames you want to use.
Check the TUTORIALS tab at the top of this page for the Animated Theme tutorial for further instructions.