Using CSS :target with Javascript

Not sure if this can be done or not, but I'm banging my head trying to figure it out. So I have a pure CSS modal that utilizes :target to fire. However, I would like to use Javascript to add/remove a class to my Body when it is fired so I can prevent page scrolling behind the modal.

[data-am-modal] {
  position: fixed;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  overflow-x: hidden;
  overflow-y: auto;
  -webkit-overflow-scrolling: touch;
  z-index: 999;
  width: 0;
  height: 0;
  opacity: 0;
}
[data-am-modal]:target {
  width: auto;
  height: auto;
  opacity: 1;
  -webkit-transition: opacity 400ms ease;
  transition: opacity 400ms ease;
}
[data-am-modal]:target .modal-overlay {
  position: fixed;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  cursor: default;
  background-color: rgba(0, 0, 0, 0.4);
  z-index: 1;
}
[data-am-modal] .modal-dialog {
  position: relative;
  width: calc(100% - 30px);
  max-width: 600px;
  margin: 15px auto;
  z-index: 2;
}
[data-am-modal] .modal-content {
  background-color: white;
  padding: 30px;
}
[data-am-modal] .modal-close {
  position: absolute;
  top: 15px;
  right: 15px;
}

/* Base Styling */
body {
  margin: 0;
  font-family: 'Open Sans', sans-serif;
}

.link {
  display: inline-block;
  padding: 15px;
  background-color: #1e91c7;
  color: #fff;
  border-radius: 5px;
  text-decoration: none;
}

<a href="#modal-1" class="link">Open Modal</a>

<p>
filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>filler text<br>
</p>


<div data-am-modal id="modal-1">
    <a href="#!" class="modal-overlay"></a>
    <div class="modal-dialog">
        <div class="modal-content">
            <a href="#!" class="modal-close">Close</a>
            <h1>CSS Modal</h1>
            <p>Cras mattis consectetur purus sit amet fermentum. Cras justo odio, dapibus ac facilisis in, egestas eget quam. Morbi leo risus, porta ac consectetur ac, vestibulum at eros.</p> <p>Praesent commodo cursus magna, vel scelerisque nisl consectetur et. Vivamus sagittis lacus vel augue laoreet rutrum faucibus dolor auctor.</p> <p>Aenean lacinia bibendum nulla sed consectetur. Praesent commodo cursus magna, vel scelerisque nisl consectetur et. Donec sed odio dui. Donec ullamcorper nulla non metus auctor fringilla.</p> <p>Cras mattis consectetur purus sit amet fermentum. Cras justo odio, dapibus ac facilisis in, egestas eget quam. Morbi leo risus, porta ac consectetur ac, vestibulum at eros.</p> <p>Praesent commodo cursus magna, vel scelerisque nisl consectetur et. Vivamus sagittis lacus vel augue laoreet rutrum faucibus dolor auctor.</p> <p>Aenean lacinia bibendum nulla sed consectetur. Praesent commodo cursus magna, vel scelerisque nisl consectetur et. Donec sed odio dui. Donec ullamcorper nulla non metus auctor fringilla.</p> <p>Cras mattis consectetur purus sit amet fermentum. Cras justo odio, dapibus ac facilisis in, egestas eget quam. Morbi leo risus, porta ac consectetur ac, vestibulum at eros.</p> <p>Praesent commodo cursus magna, vel scelerisque nisl consectetur et. Vivamus sagittis lacus vel augue laoreet rutrum faucibus dolor auctor.</p> <p>Aenean lacinia bibendum nulla sed consectetur. Praesent commodo cursus magna, vel scelerisque nisl consectetur et. Donec sed odio dui. Donec ullamcorper nulla non metus auctor fringilla.</p>
        </div>
    </div>
</div>

https://jsfiddle.net/gpjcauye/6/