.top-menu > .menu-item:hover:after, .top-menu > .menu-item.active:after,.top-menu > .menu-item.current-menu-item:after { border-bottom: 2px solid #000; content: ""; height: 1px; left: 0; position: absolute; top: 39px; width: 100%;}
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go to file ..\wp-content\themes\chillout\style.css
line - about 1125 - there You have css code: modify border-bottom color, and voilà
All the Best.