Ahmad Shadeed shows us how to stylize scrollbars in CSS.
See the Pen
Custom Scrollbar – 2 by Ahmad Shadeed (@shadeed)
While I’m no big fan of doing so in general, I can see some specific situations where it might come in handy, such as styling the scrollbars of overflowing tab button bars.
Before you go all-in on this, you might also want to read Baseline Rules for Scrollbar Usability