Very nice and to the point game with a clean UI! I love the pixel art graphics, sounds and music. Anyways, I think you will already be adding this in a later update, but still I wanted to say the game doesn't save the progress of levels right now but no issue, I am sure it can be implemented easily.
Also, when I quit the fullscreen using ESC (I didn't try using F) and then came back to the game, for some reason, it was registering my mouse clicks but when I started the game, I could not jump by pressing Space.
Overall, really nice game!! Well done!!! =)