@exoplanet When you install the browser extension for the first time, it adds a lamp button that dims the entire page. If there's a video on the webpage, it brings the video to the top of the dark layer.
I (developer of Turn Off the Lights) believe in customizing the web to suit your preferences. On the Turn Off the Lights Options page, you can enable the Night Mode feature. Additionally, you can customize the lamp action to activate the Night Mode feature.
See this YouTube video tutorial (you see the Chrome web browser, but it is a browser extension that works on all web browsers):
https://www.youtube.com/watch?v=0cTEUE2lm24