The theme is very minimal. I think the premise is “someone is trying to sneak a letter to the princess. Guess who has it? The guard? The maid? Etc.” It’s just a deduction game.
But there are also loads of “themed” versions if you prefer. Batman, Cthulhu, Munchkin, Santa Claus, Legend of the Five Rings, The Hobbit, Marvel, etc.
And since there are only 13 cards, you could easily re-theme it with a Deck of Many Things or something.
Love Letter. A very quick game with just 13 cards. Games take about 3 minutes so you can play multiple rounds if you want. Suits 2-6 players (best at max 4 in my experience). Generally very popular and easy to learn.
Comes in dozens of themes as well, if you don’t like the “princess in a castle” theme. You can find Batman, Cuthulu, The Hobbit, versions depending on your preference.