'Close this personal chat' button: blue hover misaligned

Hey guys, I noticed this on a forum I’m on. In the sidebar where you can see all your PMs at a glance, the x button next to the name of the user has a blue rectangle when hovered. The thing is, the blue rectangle is not centered over the x.
Is this a theme issue, or is this a Discourse thing?

Looks like a theme issue on Ask, because on other themes the rectangle is the same color as the background when you hover.