Have you tried using the Image Intensifer or the flares?

That's AvP for you :P

Joking aside: The pitch black darkness is an important gameplay element, so it's working as intended. Press "Q" to switch through the vision modes of every species. They all have a vision mode that allows them to see in the dark - but those vision modes come with some disadvantages too! The Marine (for example) is easily blinded by light sources when playing in vision mode for darkness.

Another possibility, when you're playing as marine, is to hit "F". This'll throw a flare, illuminating dark areas and blinding Marines with activated image intensifier (= night vision equipment).

I believe in the options there is a way to turn the brightness up or down by adjusting the gamma, if that is what you would like to do.

EDIT: Oops, sorry I did not fully read the post XD

I know it isn't helpful, but the game is supposed to be stupidly dark.

Yes, and it's kind of weird how they did it. After loading a mission or a map hit escape, scroll down to Audio/Video Options and slide Gamma Settings all the way to the right, scroll down to Use These Settings and hit Enter. The "Use These Settings" part always throws people off. For some reason the gamma/brightness option is not available until after you start playing.

If you want to try a more complicated method (which I think is what you're looking for) AvP Classic supports ENBSeries. With it you can manipulate all sorts of things including brightness, contrast, bloom, etc..