<style>
::-webkit-scrollbar {
    background: red;
}
body {
    overflow: scroll;
}
div {
    overflow: scroll;
    width: 200px;
    height: 200px;
}
</style>
<div></div>
<script>
window.onload = function() {
    document.body.offsetWidth;
    document.styleSheets[0].cssRules[0].style.backgroundColor = 'green';
}
</script>