You can use the ::webkit-scrollbar selector. Below is an example, but the link has much more!
Just place this code in userstyle.css (and userchrome.css if you want it on all scrollbars)
::-webkit-scrollbar {
width: 20px;
}
::-webkit-scrollbar-thumb {
background-color: white;
}
There is a wiki for sharing customizations as well as a thread for sharing. The wiki includes a link with an introduction on how to discover css classes to target for customization, but ultimately you'll need to either know css or refer to customizations that people share.