I'm sorry what do you mean by 'the light started to get much darker'? u mean the 'fade to black' transition after clearing the level? 'After cleaning the level it still was dark.' sorry i don't understand... do you have any screenshots?
I mean the whole light level of the game got dark. It's like you put a dark texturerect with some transparency on top of the game. I don't made any screenshot because I was not sure if this is intended.
ah i think i know what happened, no thats not part of the game... i think u just highlighted the game by accident using ur mouse... the same result could be replicated by pressing the game page and pressing ctrl+A.