Under Graphics, you can alter the Bloom Amount. It'll be under Advanced Graphics Settings. That will only darken or light up the world a little though. To really see a change you'll have to alter the Gamma. It'll also be under Graphics, and it should be under the first section, also titled Graphics. The problem is that if you take screenshots, the effects won't be captured in them. It's still handy for actual gameplay though.
Hope that helped somewhat.