iDVD is designed as an authoring application, but its options are limited and built-in. If you need more versatiIity, and are willing to spend time learning a new method, you may want DVD SP.
For your current question, it doesn't matter what theme you initially select, if you don't want one, set your project to have the DVD play immediately. No menu/theme will be seen.
Go to your iDVD project's Map view and drag your iMovie from its current box into the first one that says to 'drag content here to automatically play when disk is inserted' and you rmovie will start right away as if it has no menu.
When the movie is finished, the themed menu will come up unless you set the movie to loop, or devise a black screen--see these for ideas:
http://homepage.mac.com/profpixel/AutostartDVD/autostart.htm
http://discussions.apple.com/thread.jspa?messageID=10230676�