You can't style a scrollbar (other then to turn it on and off) with CSS at all.
There is some proprietary stuff which lets you apply some styling, but this is supported only by IE and Opera.
Chrome provides no mechanism to do this.
As a commenter points out, WebKit now supports a different proprietary mechanism for styling scrollbars. I've no idea if the Chrome build of WebKit has this merged or enabled though.
You could look at replacing the scrollbar wholesale with JavaScript, and jScrollPane appears to do a reasonable job of not breaking the usual interaction rules.
That said, changing the appearance of user controls is something I'd try to avoid, and making something users need to aim a pointer at smaller sets off the flashing red light marked "Fitts's law".
A better solution would probably be to "Not cram so much information into so little space".