From the YouTube Blog:
Starting today, we’re opening up a preview of the new design to a small group of people from all around the world so we can get feedback. While we hope you’ll love what we’ve been working on, we’re also really excited to involve the YouTube community so we can make the site even better before sharing it more broadly.
The site design is built on a new, faster framework named Polymer, which enables quicker feature development from here on out. And today, we are introducing one of the first new features developed on Polymer: Dark Theme. Developed to cut down on glare and let you take in the true colors of the videos you watch, Dark Theme turns your background dark throughout your entire YouTube experience.
If you want to try out YouTube’s latest look, you can opt-in to preview the new design at youtube.com/new. You can return to the current design by selecting “Restore classic YouTube” from the Account Menu. And don’t forget to send us feedback from the Account Menu.
Learn More About the New Look and Feel