If you want to activate new You-tube design,
here the trick for you:-
- If you use Google Chrome, press 'Ctrl + Shift + J' or 'Ctrl + Shift + K' in Firefox.
- Click on 'Console' tab and copy & paste the given code at end of page.
document.cookie="VISITOR_INFO1_LIVE=ST1Ti53r4fU"; - Hit enter. Make sure that you see the given script written in Console tab.
- Reload the page.
That's it. Enjoy new YouTube layout!