A theme can be an underlying topic of a discussion or a recurring idea in an artistic work. Anxiety about getting married is a big theme in romantic comedies.
From ancient Greek thema (via Latin and French) we get this word, whose many related uses all have to do with the idea of "the main subject of something." In music, it can be a recurring melody in a composition, or an entire song throughout a play or film (a theme song). It can also describe the decor of a setting: "The amusement park had a Wild West theme."