What is the difference between Themes in iMovie '11 and Themes in iDVD?

I need to get something straight in my mind because its been 4 years since I've made an iMovie and I'm now embarking on a new, long project. I am reading The Missing Manual for '09 and much is coming back to memory from earlier editions of iMovie, but my question is not addressed in that book. I'm sure that some of you experienced iMovie users can explain what happens to the Theme that one selects in iMovie when you export to iDVD and are then supposed to select a theme? Thanks for the help.`