What system version are you running?
First log into another user account on your Mac and see if the theme problem exists there.
If so go into the HD/Library/Application Support/iDVD folder and make a backup copy of the entire Themes folder. Then enter into Time Machine, go back in time to before the problem started and restore the Themes folder. Repair disk permissions and try iDVD again. You may have to go into iDVD's Advanced preference pane and confirm the location of the themes:
Thanks Old Toad,
The themes display correctly on another, already installed, user account. Then I saw that the themes were also present in the Application support of my user account library. I deleted from my library the themes, the cache files and the 2 pref files com.apple.iDVD and com.apple.iDVD.LSSharedFileList. That cured the problem!
My system is 10.6.8. (The machine is mainly used for FCP7, so I am staying with snow leopard and am perfectly happy.)
Thank you for your help and long live iDVD! The whole world is not yet in the Cloud.