Resize the windows and get a stylish scroll bar for the menu

what parameters do i have to add in my NavBar.css (i.e menu in left side) if I want to see a scroll bar (in transparent white) appearing when i resize my windows (i.e I can scroll down or up if the window is not at the max so I can still scroll the menu and see the other icons) please ?

.NavBar{
  width: 15rem;
  height: 100vh;
  background-color: #FF7F50 ;
  flex-direction: colums;
  gap: 1.25rem

}

This is what i want to have not in green but in transparent/grey bar .

enter image description here