Custom scrollbar only in one div

If you are doing it for a class, you can do it in the following way. Side bar is the class given to the div that you want to allow scrolling.

.side_bar{
    padding-right: 20px; 
    margin-left: -12px;
    position: sticky;
    top: 0;
    height: 100vh;
    overflow-y: scroll;
    /* width */

}
.side_bar::-webkit-scrollbar {
        width: 5px;
    }

    /* Track */
.side_bar::-webkit-scrollbar-track {
        box-shadow: inset 0 0 2px grey; 
        border-radius: 10px;
    }

    /* Handle */
.side_bar::-webkit-scrollbar-thumb {
        background: rgb(7, 7, 7); 
        border-radius: 10px;
    }

    /* Handle on hover */
.side_bar::-webkit-scrollbar-thumb:hover {
        background: #009eb3; 
    }

#boardslist {

  &::-webkit-scrollbar {
   width: 0.5em;
   height: 0.5em;
  }

  &::-webkit-scrollbar-thumb {
   background-color: rgba(255,255,255,.1);
   border-radius: 3px;

   &:hover {
    background: rgba(255,255,255,.2);
   }
  }
}

Check this out http://codepen.io/tholman/pen/tldwm