Thanks for playing! You actually can still use the arrow keys to get to the exit button in the shop (just keep pressing down), or you can press escape to close it. Probably more of an issue with how the buttons are laid out- having the close button off to the side kind of makes it look unrelated to the other buttons.
Good note on the background too- I think there may have been a cleaner way to get the sort of effect I wanted. Maybe just tiling the background so it could move with all the foreground elements but keep the background drawings in view would have been enough.