<script type="text/javascript">
let btn = document.getElementsByClassName('slicknav_btn');
let tohighlight = document.getElementsByClassName('slicknav_nav');
let allothers = document.getElementsByTagName('div');

btn.addEventListener('click', function(){

  for(let i = 0; i < allothers.length; i++){
    allothers[i].style.opacity = "0.3";
    allothers[i].style.background = "#dedede";
  }

  for (let i = 0; i < tohighlight.length; i++){
    tohighlight[i].style.opacity = "1";
    tohighlight[i].style.background = "#98FB98";

  }
});
</script>

<div id="slick-mobile-menu">
<div class="slicknav_menu">
<a href="#" aria-haspopup="true" role="button" tabindex="0" class="slicknav_btn slicknav_open" style="outline: none;">click me</a>
<div class="slicknav_nav"></div>
</div>
</div>