I would say the typical theme for pop music would be LOVE. You could really hear examples of this music virtually anywhere, examples such as "that's what i like " by Bruno Mars, "stay" by Alessa Cara, or "Despacito" by Luis Fonsi, Daddy Yankee, and Justin Bieber. I think this theme is such a popular one because people really like to sing or rap about their feelings towards their lover because it's fun and people like it. Plus it appeals to some of those people who are loveless and are trying to find love in a world full of lovers. Lastly, i think it appeals to those teenage girls out there who love singing along to their FAVE boy singer or girl singer.
Hope this answer helps you :)