f.lux is makes the color of your computer's display adapt to the time of day, warm at night and like sunlight during the day.
Darkness is a Chrome extension that provides dark themes for popular websites
The entire page will be fading to dark, so you can watch the videos as if you were in the cinema. Works for YouTube™ and Beyond.
With a single click on the lamp button, the page will fade to dark. And automatically focus on the video. By clicking on it again, the page will return to normal.