To delete a customized theme:
Quit iDVD.
Locate the Favorites folder on your hard disk, where the customized theme you want to delete is located:
If you selected the “Shared for all users” checkbox in the “Save as Favorite” dialog, the theme is saved in this location: /Users/Shared/iDVD/Favorites.
If you did not select a checkbox or you selected the “Replace existing” checkbox in the “Save as Favorite” dialog, the theme is saved in this location: /Library/Application Support/iDVD/Favorites.
Drag the theme you want to delete to the Trash.
It immediately becomes unavailable to iDVD.