As noted in a Knowledge Base article, themes from iDVD 1 and iDVD 2 are unavailable in iDVD 4 unless you upgrade from iDVD 3. MacFixIt reader Rich Grula, who did a "clean install" of iDVD 4, discovered a frustrating consequence of this issue:
"When I tried to open an iDVD file created using an original iDVD theme, iDVD tells me that the Baby Blue theme can't be found and it will substitute the current default theme instead. The bigger problem is that this screws up the original themes I created using the shell of the iDVD 1 and 2 themes. I used to choose those because they were the most basic and had no motion menus or drop zones. I'd drop in my own photos. Now I discover I'll have to rebuild all those DVDs."
The Knowledge Base article above does note that "projects created in iDVD 1 and iDVD 2...will use the iDVD 4 default theme," but many users don't discover this until after they've tried to open their older iDVD projects in iDVD 4.