Board Thread:Support Requests - Getting Technical/@comment-24505665-20140411222331/@comment-11733175-20140412073219

It's done with transitions. You set up the un-hovered styling as it appears normally, then add some different styling for the hover: background-color, color, text-shadow, etc. The transition is the part that makes it gently change as opposed to instantly changing. You can see it in use on Bleach Wiki.