Every dungeon has a different level of ambient brightness. Some glow green, others have a deep blue hue. However, some are just plain black with no ambient glow. That's the way the game was designed. The screenshot you posted looks fine on my system, however you
are standing on a dark ground with the dark gray dungeon walls around you. That is inherently one of the darkest sections of the entire game, so you chose the most appropriate place in the game to point out your frustration. Most other dungeons, with light colored floors and walls, appear brighter.
Are you saying you can't enjoy the game because you expect to see more of the dungeon than what is visible around the radius of your torch glow? You shouldn't be able to. Your character's ability to illuminate the dungeons through torch, lantern, and magical light is part of the game's strategy.
There are plenty of 3rd party gamma adjusters that you can download to temporarily boost the gamma of the game. Here's one I found for someone else:
http://files.filefront.com/Gamma+Adjust ... einfo.html