Adding a line below nav

Site URL: https://mauve-crane-alan.squarespace.com

Hi I've added a key line below the nav using CSS. But I want the line to extend to the past the site padding to the left and right edges. 
 

This is the code I've used so far:
.header-title-nav-wrapper {
    border-bottom: 2px solid white;
    padding-right: 1vw;
    padding-top: 1vw;
    padding-bottom: 3vw;
}

19 hours ago, Jordanheid said:

Site URL: https://mauve-crane-alan.squarespace.com

Hi I've added a key line below the nav using CSS. But I want the line to extend to the past the site padding to the left and right edges. 
 

This is the code I've used so far:
.header-title-nav-wrapper {
    border-bottom: 2px solid white;
    padding-right: 1vw;
    padding-top: 1vw;
    padding-bottom: 3vw;
}

You can replace the code with this

header {
  position:relative;
}
header:before {
  position: absolute;
  width: 100%;
  content: '';
  height: 2px;
  background-color: #fff;
  bottom: 2vw;
}

Thanks!