i would like add navigation line to show which page you are on such as the one below - brooklyn them

[7kingodmsla]

here is example store

my store

[Denishamakwana]

please add below css in bottom of assets/theme.scss.css file

li.site-nav__item.site-nav–has-dropdown.site-nav–active a.site-nav__link {
border-bottom: 2px solid #000;
}
add this css first then set margin of border top or bottom as per your site
Thank you.