Maybe you went to a high school dance once. They probably had a theme. Was it “Starry Night,” “Carnival Celebration,” “The Great Gatsby,” or the classic “Confused Teenagers in a smelly Gym?”
Things get themed with holiday ornaments, the seasons, or historical eras. You’ve likely been to a “theme park.” Here’s the Wikipedia for Theme and more specifically the page for Theme (computing). Our work is about story-telling and that involves words and images and sometimes sound and video. By choosing a ‘theme,’ you can unify the look and feel of your project. It’s really just “making clear decisions” for the most part.
A theme is the set of decisions. To theme something is to actively design or implement those decisions.