L4D Reduce sky brightness or add new, dark sky

The original skies are very light. There isn't a night sky and there isn't a night campaign in the whole game.



Here is an example:



(http://www.l4dmaps.com/details.php?file=8833)



This campaign has a very nice dark sky:

http://i51.tinypic.com/140xedu.jpg

http://i54.tinypic.com/f3axzm.jpg



It would be nice if there is a method to reduce the brightness of the original skies or add a custom, dark sky. I can change the sky with Stripper:Source (only a few skies that are already included in game), but how to handle their brightness?

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Orignal From: L4D Reduce sky brightness or add new, dark sky