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.
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.
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.
Not enough for me.
I use the Safari reader, whenever possible with dark background, but not all sites are compatible with it and I loose some site experience.