Menu with underline

Hi @san2102,
Here is the solution. Just put bellows css to your end of the style.css file or Appearance > Customize > Additional css area

ul#primary-menu li a {
border-bottom: 2px solid#fefefe;
padding-bottom: 10px;
}

.gutenbiz-main-menu > ul > li > a:after, .gutenbiz-main-menu div > ul > li > a:after {
bottom: -2px;
}