Does Anybody Know A Program To Dim The Screen Lights While Watching A Video

Sometimes I don’t want to use the fullscreen option but is annoying to watch a video with different lights and colors from the other windows and alike.

I used to have one but I changed computers and forgot about the name. If I’m correct the tool worked pressing F12 and as some YouTube videos have all the screen gets dark besides the video area.

Thank you for your time.