I was searching for an answer to this question. When i found this i was like.... "someone finally asked the question." But the answer was a bit vague.
So to be specific cause I was kinda confused by this as well.......... go into your buttons inspector/ control/ custom styles...……. enable "focus" or any other of the 5 options and set "null" too "style-box-empty.
This gets rid of the annoying box that's visible in the back or front.