There are many different themes in one movie, tv show or book. There can't really be just one. Take the Book Thief for example, it has many themes like: ====================>
There are thousands of themes, however most movies, books or tv shows share several themes. So what is a theme? You may think it's things like romance, comedy or action. If you thought that, you would be wrong. Those are genres which are entirely different from themes. For example "Finding Nemo"'s genre would categorize into family or adventure, but it's themes are trust, desperation, safety, separation, discovery and so on. A theme is a moral or lesson underlying the plot. They are usually relatable to our lives which is the reason they keep coming back in the stories we love. The reason why we normally like movies, books, tv shows, etc, is not only because of the plot line but all because of the themes. As we can relate to them, we enjoy the scene. Maybe this is also the reason why most movies are similar in a way - they could all share the same themes.
Some of my favourite movies and their themes:
See if you can guess the movies from the themes and design. Comment below, if you know!