The AdSense ad code must be pasted, without modification, into websites.
There have been other discussions on altering the appearance (ie. background color, etc) and I think everyone mostly agreed it would be a quick way to get the dreaded email from AdSense. I am not even sure if there is a way to alter it using css, since I am by no means an expert on css.
AdSense might offer a wider variety of ways to display their ads in the future, which will give more flexibility to publishers, and to allow the ads to match each individual site in style.