title: How to change Scrollbar's thickness?
body: <p>Scrollbar widget in Flutter seems to have a fixed thickness of 6. I think the Scrollbar looks ugly, it is too thick. Also if I compare Flutter Scrollbar to scrollbars in other apps in my device, they all look different (they look better).</p>
  
  <p>Can I change the Scrollbar thickness without creating a new Scrollbar widget?</p>
  
  <p>I had an idea to force Scrollbar partially out of screen to make it look thinner, but I don't know how to do that.</p>
  
  <p>I am using the Scrollbar with a ListView.</p>