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Here's how to enable it in Safari (MacOS):

You need to have Developer menu enabled. Go to Safari -> Preferences -> Advanced tab, and enable Developer menu.
  • Go to www.youtube.com and from the Developer menu select Show error console (or press: option + command + I)
  • In the error console open Storage tab
  • In the left panel select "Cookies - www.youtube.com"
  • Look through the cookies and see if there is one called "VISITOR_INFO1_LIVE". If there is, delete it.
  • Then enter the following command to the error console at the bottom where it reads ">"
Code:
document.cookie='VISITOR_INFO1_LIVE=fPQ4jCL6EiE;domain=.youtube.com;path=/;max-age='+100000000000+';';

After that refresh the youtube page and press on the profile icon. That profile menu should now look completely different than how it was before. You should find the Dark Mode from there.
 
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Reactions: MrGuder
^^thanks, do I need to be signed in to u tube and if so will it save this in a settings and remember.
 
That cookie lasts until 2085. After that your browser will revert back to normal YouTube theme. ;)

But it's per browser. You need to do that with every browser you use.
 
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Reactions: MrGuder
I wish all websites and OSS were dark. Sick of burning my eyes on white screens

Ah…finally! Watching videos on YouTube and skimming comments is something I do mostly at night with tired eyes. The white screen blasting light into my face is something I prefer to forgo. Thumbs up to dark mode.

Same here, except it happens to e all day long.
My monitors brightness settings are at minimum.

You people should consider lowering the color temperature of your display. You can do that in System Preferences -> Display -> Color -> Calibrate. "Normal" color temperature is 6500K, but if you want your display to be more eye friendly, you should consider something between 5000K and 6000K.
 
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