How to reverse UI change

Download the extension called stylus

Firefox: https://addons.mozilla.org/en-US/firefox/addon/styl-us/

Chrome: https://chromewebstore.google.com/detail/stylus/clngdbkpkpeebahjckkjfobafhncgmne?hl=en

Then go to the youtube homepage and click the extension.

A menu that says "write style" should open.

Copy and paste the following in the code block.

https://pastebin.com/tmquLdGM

And press save. Should reverse it to the old version.