There was a rule that says that you can use only your 2 colors without alpha. I'm sure that you can't use effects like light in this project. That's why this jam is hard, you should find right ways of comparing colors and styles. I think your style is great without effects
I have a little problem with understanding the meaning of word "thumbnail". But I think that this:
"You can not use alpha in your art files, but you are allowed to change the alpha when the game is running "
means no. As a digital artist I can say that blurred files contain alpha and you can nothing to do with this, only use colors and pixels differently