That's perfect, thanks! I usually resort to Firefox's userContent.css file, but the feature is now disabled by default, so I'm glad to hear uBO can do it too.
Did GitHub only test their new layout on 4/3 screens? It looks so odd when the project header is fluid but the project itself is still centered and 1280px wide. Especially if you log out of GitHub, in which case the GitHub banner is no longer fluid and makes the project header look really out of place[1].
A 13" MacBook would have an effective horizontal resolution equivalent to 1280 pixels. It's likely that they designed this only for 200%-scaled laptops.
Did GitHub only test their new layout on 4/3 screens? It looks so odd when the project header is fluid but the project itself is still centered and 1280px wide. Especially if you log out of GitHub, in which case the GitHub banner is no longer fluid and makes the project header look really out of place[1].
[1] https://imgur.com/3wOE7Nr