Add a line between navigation dropdown item

You can use this code to Custom CSS box

div.header-nav-folder-item {
    border-bottom: 1px solid #000;
}

Result

If you want width of text = width of line, you can use this CSS code

div.header-nav-folder-item a {
    display: inline-block !important;
     border-bottom: 1px solid #000;
}

Result

If you want line appears on hover only, use this CSS code

div.header-nav-folder-item a {
    display: inline-block !important;
     border-bottom: 1px solid transparent;
}
div.header-nav-folder-item a:hover {
    border-color: #000;
}

Result on hover