Chrome/Firefox: Your monitor is basically a lightbulb, which is why white backgrounds are hard on the eyes. Dark Reader is a browser extension that turns bright websites dark, sparing your retinas.
Dark modes are in vogue right now: Windows 10 already has one and macOS is getting one later this year. But websites have mostly stuck to the black text on white background paradigm brought to us by paper.
Dark Reader basically inverts colors, but you can customize things a little on a per-site basis, as shown above. You can also create a list of sites that aren’t affected. Give it a spin, because it really does make things easier on the eyes.