It's pretty slick and includes a new dark mode: -
It doesn't look much different in that image, other than the dark mode, but there are quite a few changes.
To opt in, just go here: -
To enable the dark mode after opting in, just click your avatar at the upper-right and you'll see the option there.
This is only available to a limited number of users and once that number is hit, YouTube will lock it down. Eventually this will roll out to everyone.
Oh yeah - you can opt out by clicking your avatar and choosing "Restore classic YouTube".