To adjust spacing between nav items – underline

You can use this code to Website Tools > Custom CSS.
body:not(.header--menu-open) .header-nav-item--active>a {
background-position: 0 21px !important;
}

Result

To adjust spacing between nav items – underline

You can use this code to Website Tools > Custom CSS.
body:not(.header--menu-open) .header-nav-item--active>a {
background-position: 0 21px !important;
}

Result
