• YouTube recently added ambient mode to its full screen video player, which complements its new rounded video player design.
• Ambient mode works by illuminating a glow around the video window, similar to LED backlights for TVs, to create a more immersive experience for users.
• A recent update has made ambient mode more vibrant on Android devices, extending the glow below and above the video player, but it can be disabled in settings if not preferred.

And the worst part (to me):

if a video is not formatted to take up your entire screen, an ambient glow has been added to the black filler bars