Super Dark Mode for Chrome and Firefox is a free extension that allows you to apply a dark theme to websites.
It can be useful in helping you conserve battery life as well as eyestrain during long browsing sessions. We noticed that some overlays do not render correctly, making certain webpage icons/images incomplete or obscured. Still, the main body of the page does not appear to be affected.
Super Dark Mode for Chrome and Firefox doesn't take much to set up and does include some presets (filters) that you can use to customize it to specific sites and pages like Youtube, Facebook, Twitter, etc. Of course, you can turn it on/off for all if you prefer right from the top rich extension icon. It also permits whitelisting to keep them separate from any changes you make, keeping them displayed in their default state.