Last I knew (have not kept current on web specifications for several years now) changing the scroll bar of a Browser window is not supported via css as it is/was not considered "legal" according to W3C.
At least at one time, many browsers would not let a web page change the scroll bar color.
Consider that this is something an individual user can set via their browser. You have control over what your web pages look like. Users have control over what their browser looks like.
Further, many have specific color schemes for visibility reasons (think color-blind). If you could change the browser behavior, you'd be messing with such folk's ability to see their browser controls. Not a good thing.