I like your modifications; I think that you're headed in the right direction! The best advice that I can give you in terms of level design would be to make sure that each level has some sort of unique identifier. New types of challenges, enemies, obstacles, or powerups can massively help with this. Putting some time into playtesting and differentiating your levels (to figure out, from a player's perspective, how the level plays) can also give you some direction in how to design things.
My only piece of advice would be to not just change the colors of things, but to try your hand at making some new art assets altogether to better reflect your custom theme and game - make it you!
Good luck, I'm looking forward to seeing what you create!