Just about every Google app on Android devices now has a material design interface, and some of Google's web pages are using material design as well. One of Google's most popular websites, YouTube, is not part of the collection of sites which has material design elements implemented. That could be changing soon, though, as YouTube now appears to be testing a new material design interface with a small number of users, complete with deep, colorful headers that match with the color scheme of channels and other material design features. Although the material design interface is still in testing and there's no information it seems on when Google plans to roll it out to the public, it is entirely possible to enable the new interface if you're curious to check it out.
First things first, you'll have to be signed out of YouTube for the changes to appear, and head to this web address http://www.youtube.com/?gl=US, although it's much simpler to just open up a link to it in incognito instead of signing out, which is actually the only way I personally found it would work when following the steps we'll be listing below. After you've opened up the YouTube page with an incognito tab, open up the developer tools by pressing and holding down ctrl, shift, and i.
With the developer tools open, you should see a number of tabs up at the top called elements, sources, console, timeline and more, and a little arrow to the right of the timeline tab which reveals more tabs. Click the arrow to find a dropdown with the resources tab, and click resources. Here, you should see a list of options on the left. Find and expand the "cookies" option by clicking the arrow next to it, then click on the first cookie that says youtube.com. From here, find and right click on the VISITOR_INFO1_LIVE cookie, then delete it. Next, go back to the console tab and "define" the VISITOR_INFO1_LIVE cookie with this command document.cookie="VISITOR_INFO1_LIVE=Qa1hUZu3gtk;path=/;domain=.youtube.com"; (basically just copy and paste it) then refresh the page, and that should bring up the new YouTube interface with material design on it. You can also see a few screenshots below of what it looks like if you don't care to go through all those steps.