Q: iDVD center stage theme

You need to go to the Button section at the right and select the type of button that includes the video preview.

Go and change the type of button to something else, close iDVD, reopen and change it back. 

A second approach is to create a new project and see if you can get the correct button to work in it.

In the Button menu select Frames and then the type of frame you want:

I see what you're referring to.  It appears that with El Capitan that theme has become incompatible, at least as far as drip zone is concerned.  There are some other themes from Version 7 that don't work right with El Capitan.