Great movie themes?

I’m hosting a the 6th annual Littlefork Film Festival (actually just a weekend of watching flicks with friends) I need to challenge my friend with a great theme that he will need to match with movies. Past themes have been brothers, time, leaks, boats, famous pairs, monster movies, etc. They can be true “themes” that run throughout the story, or genres or just something that shows up in one scene. We like to make them challenging and find unpredictable matches.