If you download the samples the site has and look at the way it is done, you will see it doesn't alter the code of the ad itself, at all.
it's just an image behind it and a little css to position it properly
Not really that different than how I built
my software site's nav, ad, and content panels with the eye popping 3D look, and the panel on the right was originally made for adsense ads, and was approved by Google. When Google originally approved my site, that panel was empty except for some text that said
(Reserved for Adsense), so they couldn't possibly think it was for something else, and it was OK with them.
As long as you do it in a way that blends with your site, and you don't include anything in the image that could be misleading, it's ok. Built into your site properly, it is a site design element that could contain anything, a site's navigation, an rss icon, an ad, content, etc...just as I did with my software site, and you can put whatever you want there.
A no-no would be having a camera site and putting a pic of a camera near the ad in a way that misleads and deceives the visitor into think the camera is part of the ad and being advertised by that ad, and if they click the ad they can buy that camera.
But nobody will ever think that if you have a drop shadow image behind an ad that clicking the ad will get you an opportunity to buy a shadow.
That's the difference and that is how you can do it without getting yourself in trouble.
The images you use behind the ads need to be VERY generic (but not necessarily boring), and not suggestive of anything that the ads could be advertising. And not specificially unique to displaying the ads or something like big flashing arrows pointing at the ads.
If you were to say display the ads within a big black diamond shape, and also have the same black diamonds for your site's logo, advertising your mailing list and site feed, and a little row of tiny black diamonds as a seperators for your content this would be acceptable and make important elements on your site pop and catch the eye.
This would be no different that using google's own color tools to change the ad you have at the top of this page to having a dark red background and a gray border, and black & white text and using their rounded corners option...it would make it pop more.
Plus, if you seriously think it could be an issue, you can always contact google, show them what you want to do, and ask if it is ok. If they tell you that it's ok, then it is, right?