In the main menu is OPTIONS, then GRAPHIC SETTINGS, there's a gamma slider to make it lighter or darker. The one thing I found has the biggest effect is not the gamma setting, but the on/off checkbox for POST PROCESS FILTERS. You can fool around in there and try different settings, the screen resolution requires exiting and restarting the game.
Here's a couple shots showing the POST PROCESS FILTERS setting on and off;
To my eye the whole game looks better with it on.