All my textures (custom maps and normal maps) have become very bright, like there is no lighting. How do I fix this?

Is fullbright on? Do you actually have lighting in your map?

mat_fullbright 0

you have run a map that has no lights. and because its not multiplayer mat_fullbright stays set at 1 untill you tell it otherwise.

Edit: Fixed. I overlooked the compile options after I had changed them in the past.

Oh, interesting, also nice avatar.