At the moment the dark theme is available on the YouTube website, soon perhaps it may be available in the apps too:

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Go to https://youtube.com/new (it can be an individual video or the homepage) and login if you have not already

Click on your user avatar in the upper right corner of YouTube
YouTube Dark Mode

Choose “Dark Theme” from the dropdown menu

YouTube Dark Theme

Toggle the switch for “Activate Dark Theme” to the ON position, the interface change takes effect immediately

YouTube Dark Mode theme enabled

Now everything on YouTube will be dark, which makes watching videos in the dark or in the evening hours a bit nicer.

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Dark Mode on YouTube enabled ...