1) Never edit the parent theme files because you will lose your changes after theme update. In case you want to edit any styles that could not be changed via theme options use the child theme and child theme style.css to overwrite the styles https://xstore.helpscoutdocs.com/article/32-child-theme .
2) Go to Theme Options > Styling > change the border-radius for the light buttons type http://prntscr.com/sjmdxr
