I do agree with ego that some screens were a tad too dark, though. This made it rather hard for me to spot the custom character in some areas, which confused me a bit sometimes. One way to mitigate this problem would be adding a subtle "glowing" border to the sprite, just enough so the silhouette can be seen when passing through those almost pitch-black areas:
Still, I don't think it's a major issue and I didn't have any trouble completing the level, so it's probably not worth editing the sprite sheet or making a separate version just for a few screens.