i dunno if this is already on the rules, but:
Themed levels:
The wrong way to do it : just the music of the level has anything to do with the theme, backgrounds are not well cuted.
The good way to do it : Tilesets and backgrounds has to be from the game, needs references of the game (if its mario, have coin blocks or something that make people remember mario) , music has to be from the game (a remix works too)