Hi there,
Go to Additional CSS field in the customizer and remove this block of CSS:
.main-navigation .menu > .menu-item > a::after {
content: "";
position: absolute;
right: 50;
left: 50%;
top: 0px;
-webkit-transform: translateX(-50%);
transform: translateX(-50%);
display: block;
width: 0;
height: 2px;
background-color: currentColor;
transition: 0.3s width ease;
}
.main-navigation .menu > .menu-item > a:hover::after,
.main-navigation .menu > .menu-item > a:focus::after,
.main-navigation .menu > .current-menu-item > a::after{
width: 30px;
}