Where did you install iDVD from?
There should be a Themes folder in the HD/Library/Application Support/iDVD folder. If you installed iDVD from an iLife disk or from a disk that came with an earlier iMac it should have installed that folder.
However, it might be in your Home/Library/Application Support/iDVD folder. Check there. See the note below:
NOTE: In Lion and Mountain Lion the Home/Library folder is now invisible. To make it permanently visible enter the following in the Terminal application window: chflags nohidden ~/Library and hit the Enter button - 10.7: Un-hide the User Library folder.
In the event that you can't find the themes folder or can't install from an installer package you may have to purchase an iLife disk to get it working.
If you have to purchase an iLife disc in order to obtain the iDVD application remember that the iLife 11 disc only provides the themes from iDVD 5-7. The Software Update no longer installs the earlier themes nor do any of the iDVD 7 updaters available from the Apple Downloads website contain them.
Currently the only sure fire way to get all themes is to start with the iLife 09 disc:
This shows the iDVD contents in the iLife 09 disc via Pacifist:
You then can upgrade from iDVD 7.0.3 to iDVD 7.1.2 via the updaters at the Apple Downloads webpage.