Customize scrollbar in Page Action for Google Chrome extension

How do I customize the scrollbar (eg. make it black and thinner) for the Page Action开发者_如何学JAVA popup for a Google Chrome extension?

For reference, here is what a Page Action is: http://code.google.com/chrome/extensions/pageAction.html

You should be using CSS3 scroll: http://webkit.org/blog/363/styling-scrollbars/

Since the popup is just a webpage.