When I move my mouse over the little inbox icon, it gets highlighted pretty well:

　[![top bar 1][1]][1]

Sure, it's half a pixel off-center, but it's pretty good.

But when I move my mouse over my reputation notification:

　[![top bar 2][2]][2]

The grey is the wrong size!  It looks weird.  Not only is it not centered, it's too thin. 

This happens in Firefox and Chrome.  It doesn't affect the mobile site.

  [1]: https://i.sstatic.net/gjbdP.png
  [2]: https://i.sstatic.net/W5fzH.png