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What do you mean by "themed game or actual game"?

It actually doesn't really matter what you program, but it matters that you program(You could also create a program that numerically solves the schrödinger equation if you want).
I just suggest games, because you can have a lot of fun programming and testing them, and fun is beneficial in learning.

The best way is taking a simple game, that is easy to play and follows simple rules. Yes Tetris is a good example, although I'm not sure if the collision detection and turning of those weird shapes isn't a bit hard to begin with.
I'd also recommend something like snake or that chrome-browser-dino-game.
Here you should not decide based on what others (like me) tell you, but on what you would like to play.
You can also try to reach for higher goals. With the right degree of simplification(like reducing 3d to 2d graphics or make far less objects/obstacles or simplify an AI to a simple mathematical function or making only square boxes in tetris) you could program a small version of every game there is, even as a beginner.
→ So just take on of your favourite computer games from your childhood and go ahead and program a (heavily) reduced version of it.

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