How to change underline size in Navigation bar

I want to change the thickness of the underline that goes on the page that is selected.

2 hours ago, KassandraD said:

I want to change the thickness of the underline that goes on the page that is selected.

If you use 7.1, you can use this custom css, change the 2px to the thickness you want

body[data-menu-overlay-theme-switcher="true"]:not(.header--menu-open) header.header .header-nav-item--active a {
  background-size: 1px 2px;
}