For the unaware, Dark Mode is a feature that essentially flips YouTube's normal white color scheme to one mostly composed of various shades of black and gray. The feature was heavily requested for mobile apps after the launch on desktop. At the time, the feature wasn't made available to all users, nor was it officially announced by the company. Although there is no way of making the app automatically switching between the dark and light themes depending on the time of the day, the addition of the new dark theme will be welcomed by many. Before the update, users only had the white background.
YouTube's update is reportedly being done to reduce glare and give the app a more "cinematic view" as its users consume videos and even movies on the platform.