YouTube has started opening up a preview of the new design to a small group of people from all around the world so they can get feedback.
The design is aligned to look and feel more like YouTube's mobile apps. The site design is built on a new, faster framework named Polymer, which enables quicker feature development from here on out. One of the biggest changes is a new "dark theme", which turns your background dark throughout your entire YouTube experience.
YouTube also said that this is just the beginning and that new features are coming soon.
If you want to try out YouTube's latest look, go to YouTube.com/new. You can return to the current design by selecting "Restore classic YouTube" from the Account Menu.