Google has been testing the Material Design desktop UI for over a year. Previously you needed to use a URL workaround if you wanted to access it. But from today Google is letting anyone in the world opt in to the new UI by simply going to https://www.youtube.com/new. Besides bringing design parity to the desktop and app versions of YouTube, the new Material Design UI offers a brand-new Dark Theme for the desktop.
[Image: Google] MG