I wouldn't recommend the CSS route as you'd be working against a core OS feature. On OS X: System Prefs > General > Show scroll bars > "Automatically based on mouse or trackpad"; "When scrolling"; "Always".
It's possible via something like [1], but the problem is you are hardcoding colors for a specific OS version. The example given in the link is a much darker scrollbar than is present in Yosemite.
I'd just go the route of labeling the example more explicitly. By the way, nice job on the site. Very snazzy, and you didn't make the mistake a lot of people make which is to hijack the native scrolling.
[1] http://stackoverflow.com/a/10100209/2609094
