- Center the headers after GitHub changed it again in July 2021

Fixed the headers not being centered

User profile page: long user statuses were getting cut off

This morning GitHub changed their UI and broke the header tabs being centered. This version fixed the headers to be centered again.

- The previous version claimed to work on gist pages as well, but it didn't. Now it does!

- The style is now applied on all of GitHub (including user profiles which were previously excluded)
- The style is now applied on gist.github.com as well
- Fix rounded corners on buttons, labels, and other elements to be less rounded like they were in the classic look
- Fix FireFox repaint issue where there was a visible flash before the new style was applied on page load

- Make buttons have bolder font
- Fix the "parent folder" (..) in file list having too much whitespace
- Make the rounded edges aroud many boxes less rounded to match the old GitHub
- Change the line colours in the file list table to match the old GitHub
- Fix issue where the first row in the file list had two top borders
- Fix spacing on header buttons (Code/Issues/Pull Requests/etc) to mimic the spacing before GitHub's update
- Repository header is underlined all the way through on wide screens

More UI fixes: based on user feedback: fix text width of issue counters, fix text width of issue labels, fix disabled buttons to not have hover effects, make header width identical to main content size