This code snippet is added to the style.css file in the child theme. This adds a customizable border below the header text in the Default WordPress 2016 theme.
/* Add Border Below Header */
.site-header {
padding-top: 12px !important;
padding-bottom: 12px !important;
border-bottom: 3px solid !important;
margin-bottom: 16px !important;
}
Refer to the above video for customization.