Scrollbars are one of those UI components that are present on almost every page of the internet, yet we (developers) have little to no control over it. Some browsers give us the ability to customize their appearance but for most browsers including Firefox it just is not possible.
Still, Chrome and Internet Explorer (yes) make it possible for us to define custom styles for scrollbars. However the syntax horribly complex, and of course, definitely not standard. Welcome to the proprietary world. This is why you might want to have a little mixin to easily customize your scrollbars. Right?