Horizontal Scroll Bar Width

I would like to have a fixed width for the horizontal scrollbar (read: track), which is narrower than the actual width of the scrollable content (center alignment). I want to achieve this with the webkit scroll module.

I know this pushes the borders of the scrollbar, but it would be nice to achieve in terms of design.

My attempt so far: http://jsbin.com/ohusu3/5

I tried applying the minus left and right margin to the div #contentto pull it out of the scroller, but the overflow is hidden, obviously using the property scroll. I think this is possible with multiple containers. Somehow.

I avoid any jQuery plugins.