JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-cookie/1.4.1/jquery.cookie.js"></script>
<p id="dark-toggle" >Toggle unit</p>
<p id="test" class="dark-mode">test class</p>

CSS

.dark-mode{
    color:#F00;
}

JavaScript

$('#dark-toggle').click(function(){
       //this gets executed each click so we need to toggle the boolean value.
      if ($.cookie('darkModeOn')==0){
          $.cookie('darkModeOn', 1, { expires: 7 });
      }else{
          $.cookie('darkModeOn', 0, { expires: 7 });
      }
      console.log( "cookie:"+$.cookie('darkModeOn')) ;
      $('#test').toggleClass('dark-mode');
  });
  if ($.cookie('darkModeOn')){
       //this is only executed on load
      $('#test').toggleClass('dark-mode');
  }