Stylish changes the CSS of Web pages. I use
https://userstyles.org/styles/145541/total-black-global-dark-theme for a
black background. I gave you that link because you were talking about
changing the "colors" for the "browser":
please share the how to change the colors/brightness for browser
You are now talking about "altering the computers' graphics settings"... what
is not more precise. Maybe you want to change the brightness of the screen
from "Brightness & Lock" in the "System Settings", or the theme for the
graphical widgets from "GNOME Tweak Tool" (in the "System Settings" as well),
or install "redshift" (from Trisquel's repository) to adjust the temperature
of your screen: http://jonls.dk/redshift/
