This user script I found is very good, this removes the rounded corners on YouTube videos. This is a great find. It also restores the dislikes button, but I am sure it does not actually work. Still, removing the rounded corners allows the fitting of more of the video in the same space. And having less padding around it as well. This makes a big difference to the enjoyment of YouTube sessions.
This therefore looks much better than the default player and greatly improves the YouTube experience.
https://greasyfork.org/en/scripts/456132-youtube-classic-remove-rounded-design-return-youtube-dislikes. Download and install this script for Tampermonkey.