How can I style horizontal scrollbar by 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.).