YouTube Guide Link: youtube.com/watch?v=QuwrJN6DC8k
The New Material Redesign for YouTube (Console Script)

Setting the Material Layout through console -- no extension needed (this may reset your language/location preferences):
1. Go to youtube.
2. Open Developer Console (usually by pressing F12)
3. Put in the script, hit Enter and reload the page.
Code:
document.cookie="PREF=f6=4;path=/;domain=.youtube.com";
To restore Classic YouTube Design just click the button in the right bottom corner.