By default, the latest version of iDVD (version 7) does not include the themes that where part of iDVD version 5 and older. However, you can still use them with iDVD 7.
Use the following instructions to learn how to download them..
1) Open iDVD and select CREATE A NEW PROJECT...

2) Go to the themes pull-down menu and select ALL...

3) Scroll down untill you see some themes that look FADED and have NO LITTLE TRIANGE next to them. Click on one of them.

4) A Dialogue window should show up. Select DOWNLOAD NOW.

5) SOFTWARE UPDATE should show up looking for new software for your computer. Once that's done, a NEW SOFTWARE UPDATE will be avaliable for your computer. Click on the INSTALL button.

Wait for the iDVD EXTRA CONTENT to finish downloading and installing itself, and that's it. You now have extra iDVD themes.
Just make sure to CLOSE and RE-OPEN iDVD after the install for the new themes to become active.