in a few levels I can see nothing, because it is too drak. When I try to change the brightness in the options it won´t get batter. Are there any mods which improve the brightness ?

Increase the gamma in the options and your monitor, see if that helps.

it is already on 100

does the gamma slider actually do something? remember, it won't work if you are running windowed, or with certain gpu drivers under win10 (you might need to disable enhanced precision in that case, see d3d_disp_no_rgb10_buf in cam_ext.cfg).