The dark mode will soon be experienced by Android users, although the release time has not been made known.
How to start using YouTube Dark mode feature
For iOS users who wish to view YouTube videos on the YouTube dark mode; launch the YouTube app and tap your account icon in the top-right corner. Tap on Settings and then toggle on Dark theme.
Just like the iOS YouTube’s Mobile App, you can activate the dark theme or dark mode on desktops and laptops by clicking your account icon in the top-right, click the Dark theme line directly above Settings and then click the toggle switch to turn on Dark theme.
You can always disable this feature in future by toggling it off.