Hi,
I am not aware of anywhere that the Scrapbook theme template can be downloaded if it does not now appear on your Mac. You can try doing an internet search to see if you can find one.
If you have an iMovie version on a Mac that has the Scrapbook theme, you can create a project using the theme, with background clips as fillers in the place of photos, and then copy the project to the library of the iMovie version that you are presently using. Then you can use the newly created project to make other projects by dragging in photos to replace the background clips. That should work, although I am not sure whether an updated iMovie library would read a project from an older non-updated library.
Safe practice is to always backup your iMovie libraries before working with them.
-- Rich