JAVASCRIPT

html iframe and JS contentwindow

var x = document.getElementsByTagName("iframe")[0].contentWindow;
//x = window.frames[0];

x.document.getElementsByTagName("body")[0].style.backgroundColor = "blue";
// this would turn the 1st iframe in document blue.

apply css to iframe content javascript

<script>
    setInterval(function(){
      document.getElementById("iframeId").classList.add('your-class')
    },500);
  </script>

Now you need to write css code with your class name. Thank you