Parallax background was cool and the cat literally giving up the ghost every time they got hit was a nice touch.
I found it ocaisionally difficult to differentiate between what was a valid walking surface and what wasn't, as early assumptions that darkened textures were walkable didn't always apply. I'd recommend a completely different set of sprites for this purpose.
The music, though clever at first, quickly loses its appeal. I appreciate the mute button :]
I nevertheless managed to go the distance and catch my fair share of mice. Fine work, and good luck! :D