Works well with Firefox and GitHub Dark

overlay-scrollbars.css

/* overlay-scrollbars
 * based on https://github.com/silverwind/overlay-scrollbars
 * Version 2.1
 */

/* AGENT_SHEET */

scrollbar {
  -moz-appearance: none !important;
  background: none !important;
  position: relative !important;
  border: none !important;
  z-index: 999999 !important;
}

scrollbar thumb {
  -moz-appearance: none !important;
  border: none !important;
  padding: 0px 0px !important;
  background: none !important;
  border-radius: 0px !important;
}

scrollbar[orient="vertical"] thumb,
scrollbar[orient="horizontal"] thumb {
  background: #898989 !important;
  min-height: 14px !important;
  min-width: 14px !important;
  margin-top: -14px !important;
  margin-bottom: 0px !important;
  height: 14px !important
}

scrollbar[orient="vertical"] thumb:hover, 
scrollbar[orient="vertical"] thumb:active,
scrollbar[orient="horizontal"] thumb:hover,
scrollbar[orient="horizontal"] thumb:active {
  background: #5E8ACC !important
}

scrollbar[orient="vertical"] {
  width: 14px !important;
  margin-left: -14px !important;
  margin-right: 0px !important;
  margin-bottom: 0px !important;
}

scrollbar[orient="horizontal"] {
  height: 14px !important;
  margin-top: 0px !important;
  margin-bottom: 0px !important;
}

scrollbar:not(#foo) {
  background-color: rgba(135, 135, 135, .20) !important
  }

scrollbar scrollbarbutton, scrollbar scrollcorner {
  display: none !important
}