Add YouTube's Background Color gradient

[NullDev]

Since our force-load of the video completely replaces the default player, there is no changing background gradient color like on the "normal" player in non-theatre mode. This is just nice-to-have and not a priority.

Shouldn't be too hard: https://www.smashingmagazine.com/2023/07/recreating-youtube-ambient-mode-glow-effect/