Mario's palette needs to be edited, in my opinion. I think you should keep his outline, but make the rest black, just like the rest of the level.
Originally posted by toad64
The background on this really needs work.. I would just scrap the entire background and start over. Instead, maybe use the regular cave background, and make everything completely black, except for a blue outline around the edges? To sorta go with what you had in the first screenshot. Can't really comment much on design because it looks like a pretty average level. Aesthetics look nice though.