Making night blue instead of just darker, that is smart. Not sure why I didn't think of this.
Currently I am defining a color pallet. Based on the feedback from RenzoCoppolla, it seems logic to go back to the basics and define a color pallet before going forward. I think to create a better visual style, I should not start with technical solutions like lighting and shadows, but start with a design vision. I have some ideas. In my mind it's all starting to form.
@TedEH: My goal is to learn howto create and market a game. I already released this game for mobile a while ago. Its not 'viral', but it is played. I rewrote, rebalanced and updated the game for desktop platforms (windows/linux/mac) to create a real desktop experience. Ofcourse I already knew the graphics are weak, but as a creator it's sometimes hard to see the flaws. I think I got used to the current visuals, after many hours of testing.