penders - 12:15 am on Dec 22, 2011 (gmt 0)
Just a thought... by including an external stylesheet in the ad itself, does that work? Or is it stripped from what is served? If this does work, can you provide a querystring to the ad that is served in the iframe? The querystring could tell the ad which (external) CSS file to include, so this could vary per website - but is ultimately controlled by the ad.
There "should" be a way...
Should there? Other than the controlled way as mentioned above, is there any difference between your scenario and serving 3rd party ads? You should not be able to modify 3rd party ads since they have essentially paid for their ads to be viewed in a specific way, the way they designed it. But this comes back to the cross-domain security issue as mentioned in my previous post.