Turning dark mode on is just a matter of some seconds, and you may check it on in the below section:
You have to visit the Youtube website for this or you may directly have access to it if you have its application on your phone.
Then sign in if it is required.
In the top right corner, a ‘Profile’ picture or icon will be visible to you.
Now in settings, you will explore the ‘Appearance’ option, tap on it and slide the switch to the right of the dark theme option.
This is the only way to how to put dark mode on youtube.