Button styling can not be tweaked in the parameters. Each social network generally sets the styles for their buttons and outside of what we offer, there isn't much that can be done there.
With that said, the SCFindUs module uses images and a stylesheet to show the buttons. Those are just links to each social network, so it's not under each social network's control how those buttons look. You can change out the images or add rounding to each image via the CSS file. To see either, look in the /modules/mod_scsocialfindus/assets directory.
I hope that helps explain, but if you need anything else, please let us know.
Thanks,
Alex