levo - 2:01 am on Jul 3, 2013 (gmt 0)
Not sure whether you can use external css. Although I suspect this is the long term plan. Looking at the Adsense JS code, it interrogates the element.style.width and element.style.height, and then constructs the ad call from that.
The code looks for the inline style to be foolproof - if the webmaster didn't remove it, external styling is ignored..