CODE EXAMPLE FOR CSS

custom horizontal scrollbar css

::-webkit-scrollbar {
  height: 4px;              /* height of horizontal scrollbar ← You're missing this */
  width: 4px;               /* width of vertical scrollbar */
  border: 1px solid #d5d5d5;
}

Since logically one cannot force a vertical scrollbar to be a certain height (since dictated by the positioned parent) 
Therefore such height property is to target the horizontal's scrollbar height 
and vice-versa (width for the width of the vertical scrollbar).