How you acquire
xgamma depends on which distro you are using - for example, on Arch I installed the packages
xorg-xgamma (official) and
To set the contrast to 70% using
xcalib -co 70 -a
To set the gamma to 0.4 using
xgamma -gamma 0.4
After you’ve fiddled with these for a while and want to reset:
To persist these settings, you could put them in your
.xinitrc depending on your window manager.
You may also be interested in the topic Colour calibration and ICC profile - interestingly, I find the contrast on my Librem screen far too high, and in this topic we are finding a way to calibrate the screen automatically to known good settings using an ICC profile file.
Until then, hopefully tweaking the settings with these tools will help.