To move iDVD themes:
Locate the Themes folder in this location on your hard disk: /Library/Application Support/iDVD/Themes.
Move the folder to a different location, such as an external hard disk.
Next, you need to set the new location of the Themes folder.
With iDVD open, choose iDVD > Preferences and click Advanced.
Click the Add button.
Locate the folder where you moved the themes, then click Open.
The name of the new folder appears in the folders box.