YouTube Dark Mode
The new design aims to be simple and consistent. It has more white space. The white background comes across as smoother and more pleasing, but it is not too different from the previous design, and the squared icons get replaced with round thumbnails.
The biggest change is that you have more control options available through your icon in the top right, and of course now you can turn on a new "dark mode," this replaces all the white UI and makes it easier on the eyes. The dark mode switch is in the new profile menu, which you can access by clicking on your profile picture in the top right.
The design isn't rolling out for everyone yet. So, if you want to try it out, go to your youtube.com and add /new - youtube.com/new
You can easily switch back to Classic Mode by clicking your profile picture in the upper right and scrolling down to select it.
YouTube says it will have "quicker feature development from here on out." It cites the dark theme as the first of these Polymer-enabled features and says, "This is only the beginning—you can look forward to more powerful new features coming soon!"
The new design looks good, but the downside so far is that enabling it will also disable your YouTube downloading extensions that you may have installed. Try it out and remember you an always switch back to classic mode. Extension developers are probably already writing new code for the new design.