<!DOCTYPE html>
<iframe frameborder="0"></iframe>
<div id = "content">
<div style="width: 400px; height: 400px"></div>
</div>
<script>
    var idoc = document.querySelector("iframe").contentDocument;
    var sheet = idoc.head.appendChild(idoc.createElement("style")).sheet;
    idoc.body.innerHTML = document.querySelector('#content').innerHTML;
    document.body.offsetTop;
    sheet.insertRule("::-webkit-scrollbar { width: 50px; height: 20px; }", 0);
    sheet.insertRule("::-webkit-scrollbar-thumb { background: green; }", 1);
 </script>