Nice entry! I have a few tips that may be worth considering.
- To easily improve the visuals of a game in a small amount of time, you can look up a color palette and use those colors.
- I think the shooting would be more fun if you could hold down the button to shoot AND if it shot faster. Using a machine gun would be more fun than a slow pistol.
- You could balance this by making enemies spawn faster, which would increase the action overall.