<!DOCTYPE html>
<style>
::-webkit-scrollbar { background-color: green; }
</style>
For manual test, move mouse over the red bar.
The test passes if the bar turns green on mouse over and red on mouse out.
<div style="width: 200px; height: 200px; overflow-y: scroll" id="scroller">
<div style="height: 500px"></div>
</div>