I assume you have a new Mac that does not come with iDVD out of the box. Follow these steps and you should be good to go:
1. Download iDVD 7.0.4:
http://support.apple.com/downloads/iDVD_7_0_4.
2. Mount the disk image you just downloaded and open it to view its contents.
3. Right-click "iDVD_704.pkg" and select "Show Package Contents".
4. Open the folder "Contents" and double-click on "Archive.pax.gz".
5. A new Finder window will open where the folder "Archive" is located. Open it.
6. Open the folder "Applications" and drag iDVD to your Applications stack. (Usually it's located in the left pane in Finder under Favorites).
7. Now download the iDVD 7.1.2 update:
http://support.apple.com/downloads/iDVD_7_1_2.
8. Mount "iDVD7.1.2Update.dmg" and open it to view its contents. Launch "iDVD7.1.2Update.pkg".
9. Install iDVD via the Installer. Once complete, try launching iDVD from your Applications stack.
10. Select "Create a New Project" and then "Create". (It doesn't matter what name the project is.)
11. You will see a lot of exclamation marks. That's normal. Now select "Old Themes" from the drop down on the right pane. (It should read "All".)
12. Select "Baby Mobile". iDVD will prompt you to download extra content. Select "Download Now" and OK.
13. Software Update will launch and check for updates. Once it's finished, select "Install".
14. This will download around 540 MB extra content, or themes. Once it's finished, quit iDVD. (You should get a prompt saying "Please quit iDVD before beginning this update." Select OK afterwards.)
15. Once the update has finished installing, select OK.
16. Now you may remove the recently created iDVD project, the "Archive" folder and unmount any disk images. (You should keep the iDVD disk images to yourself though, in case you need to reinstall iDVD later.)
17. Once you're done, open iMovie, open your project and choose "iDVD..." from the "Share" menu. Select one of the downloaded themes from the list on the right.
18. Thank me for discovering this technique just now.
Following my instructions above will install a functional version of iDVD with themes from iDVD versions 1 through 5. I haven't found a way to install iDVD 6 and iDVD 7 themes yet, but this should get you going if you don't have an iLife installation disc and purchased a new Mac with OS X Lion.