Once again google is changing the look and feel of the video hosting site youtube.
follow the below steps to enable it.
Open your browser’s development tools (only firefox and chrome)
In Firefox: Ctrl + Shift + K (Win) | Cmd + Shift + K (Mac)
In Chrome: Ctrl + Shift + J (Win) | or Cmd + Alt + J (Mac)
Then add below code to the console and hit enter.
close the browser development tool and reload the youtube page.
To go back to the old design just clear all cookies that YouTube has set on your computer.