YouTube for Android finally starts getting dark mode5
Because this is a server-side update, you won't have to download anything. You'll certainly know when the dark theme is available on your device since YouTube automatically applies it when you open the app.
The dark theme comes with a toggle that allows you to switch it off. There are no additional changes in terms of design when you apply the new theme, except for the obvious darker color, which isn't really black but more of a dark gray.