ryanrybot, thank you for advices and info I'll try to add some 'juice' , but there's an issue about loud sfx, cuz i don't know, how can volume be varied, depending on impact force. Just making it depend on velocity didn't work well, as some inconspicious impacts sounded way too loud, so I've decided to make just one volume level for all situations.
I'll try what you are saying, and will see, if it works well with the game's atmosphere.
About the background - I'll try to make it darker.