Brightness is set to full but game is very very dark. Any Solution?

You've got the GOG version, right?

Which option did you choose when setting up the graphics at the start? Aka direct3d, glide, etc.

F11 changes the brightness ingame. Also you can change the brightness via main menu, in options.

Into the the wrapoer configurators you can also modify the gamma/brightness, so the final result will depend of your brightness via wrapper and the brightness selected ingame.

Anyway I believe this game was intended to be played relatively dark, because I know there is a weapon/tool that offers light like a lamp later in the game.
There are enemies hiding in the shadows and the only thing you can see from them if you are lucky are their red eyes. It is very atmospheric. Also if the game is too bright the textures and lighting are too ugly and "banded"

So you should tweak yor brightness to what you are comfortable with and to what looks fine for the game

Greetings

Also, note that there are certain areas of the game, for example Shadar Logoth, where shadows are purposefully set to almost completely mask an enemy, except for their glowing red eyes.