I'm sorry but I don't see a border in that screenshot. Are you referring to the white round fill? The white part is the Social element's background. It has a big border radius which makes it round. You can see those if you edit each of the Social element. For more details about the Social element, please read the documentation at https://theme.co/apex/forum/t/elements-social/10241
To make it easy to replicate the setup to the rest of the Social elements, remember that you can set an Element Preset using the Template Manager.
Hope that helps.