Webkit's support for scrollbars is quite sophisticated. This CSS gives a very minimal scrollbar, with a light grey track and a darker thumb:

RGBA color values are supported in IE9+, Firefox 3+, Chrome, Safari, and in Opera 10+.
RGBA color values are an extension of RGB color values with an alpha channel - which specifies the opacity of the object, the A in RGBA..

An RGBA color value is specified with: rgba(red, green, blue, alpha). The alpha parameter is a number between 0.0 (fully transparent) and 1.0 (fully opaque).