Custom Active Nav Link

Site URL: https://sawfish-tulip-pxkb.squarespace.com/

I would like to replace the active tab underline with an image I have uploaded to my Custom CSS files. I only want it to appear when on the page and not when you hover over the links.

I've currently put in this code but can't seem to get the sizing correctly to adjust to each link and display the image at 100%. 

// active nav links
.header-nav-item:hover a,
.header-nav-item--active a {
  display: inline-block;
  position: relative;
  background-image: none !important;
}
.header-nav-item:hover a:after,
.header-nav-item--active a:after {
    content: '';
    position: absolute;
    width: 150%;
    height: 20px;
    left: 10%;
    bottom: -20px;
    background: url('https://static1.squarespace.com/static/611c2078e04ba63b7ce92922/t/611c461d505a25511e4aca07/1629242909476/Active+Link%402x.png');
    background-size: contain;
    background-repeat: no-repeat;
}

Anyone able to help?

@bangank36  I saw you answered something similar to this here - 

Hi,

What is access password?