Thanks for giving the game a shot! I didn't spend a lot of time on it, resulting in a puzzle game that's just a zen mode. If I were to go back and spend some more time on it I'd just add in a time mechanic so you need to solve each puzzle in a limited number of seconds. That way there is a way to lose, and it becomes a matter of seeing how far you can get.
As for a quit key, games exported by Godot usually have the standard Alt-F4 way of quitting built in. Works that way on windows, should on linux as well.