Some changes to the opening, like maybe adding visuals and changing the way the text is presented (in a fixed spot on the screen, and possibly a different font).
I didn't get past level one, but from the text in the beginning I think I know how you were going to incorperate the theme.
During the level, the background was a bit much stillbetter than leaving it blank though. Also making it so that you can hold down the left mouse button to rapid fire would be nice, but that might just be me because my mouse makes loud clicking noises.