edit css you have to add -moz-appearance line.
You can find details look at the link below;
Sample
https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Scrollbars
Details
https://developer.mozilla.org/en-US/docs/Web/CSS/::-webkit-scrollbar
.item-list {
  scrollbar-color: rgba(255,255,255,.5);
  scrollbar-width: thin;
}
.item-list::-webkit-scrollbar {
  -webkit-appearance: none;
  -moz-appearance:none;
  width: 10px;
}
.item-list::-webkit-scrollbar-thumb {
  border-radius: 5px;
  height: 80px;
  background-color: rgba(0,0,0,.5);
  -webkit-box-shadow: 0 0 1px rgba(255,255,255,.5);
}
If you want to show scrollbar always you have to use min-height 
css 
  .insider {min-height:250px; }
  .item-list { width: 200px; height: 200px; overflow-y: scroll; scrollbar-width: thin; scrollbar-color: rgba(0, 0, 0, .5) rgba(0, 0, 0, 0); }
  .item-list {
    scrollbar-color: rgba(255,255,255,.5);
    scrollbar-width: thin;
  }
  .item-list::-webkit-scrollbar {
    -webkit-appearance: none;
    -moz-appearance:none;
    width: 10px;
  }
  .item-list::-webkit-scrollbar-thumb {
    border-radius: 5px;
    height: 80px;
    background-color: rgba(0,0,0,.5);
    -webkit-box-shadow: 0 0 1px rgba(255,255,255,.5);
  }
<div class="item-list">
  <div class="insider">
  </div>
</div>