Do your eyes get tired reading predominantly white browser screens? Mine do, which is why I was attracted to this free add-on.
Dark Reader inverts the brightness of a screen, as these before and after images show:
The add-in is available for Chrome and Firefox. Not all screens are amenable to inversion, though most appear to be. You can choose which sites should be inverted or not, as well as adjust the criteria for brightness and contrast.
Dark Reader's interface is one of the best I've seen: intuitive, clear, and aesthetically pleasing.