That is weird. Both in Safari on macOS and iOS I get the border around the button. The CSS you are referring to, is that standard in GP? If yes, how do I overwrite it? I haven’t added anything the likes of it.
I am able to overwrite it using this less elegant solution:
Odd – GP doesn’t add anything to the Additional CSS. The comment at the end of the Additional CSS is what we add to the CSS that comes as part of a GP Site install.