YouTube's new layout put rounded corners on everything and this stylesheet will make the corners square again for thumbnails and avatars. This will also square the corners of the mini-player and certain other elements which appear on the main pages of the site. If for some reason this script has stopped working, that just means YouTube has recently changed something and I'll fix it shortly.

There will be an option to disable square avatars while you're installing it, or you can do so from within Stylus by clicking the gear icon next to the stylesheet's name.