Thank you very much for the suggestions.
I knew about mouse capturing before, but had completely forgotten about it when I made this game, so I am a bit sorry about that.
Also, I disagree that making the spacebar the only way to shoot would make the game more accessible. On the one hand, it Would make it more obvious that you can shoot with the spacebar, but this would also hinder other players who prefer to use the Mouse Button instead.
And yes, I have just kind of thrown in the theme with no real relevance to the gameplay. I have not done a game jam or anything similar before, so of course I am not very good at thinking creative fast compared to other people.