Well! Maybe the reason is that your theme delivers rather strange HTML. Right after the ad section you have an extra </head> then comes the meta tags for keywords and description and after that you seem to end the generated document with </body></html>
After this point comes the posts. This is not valid html or xhtml and the browser will be kind enough to do its best.
I think before you try to tamper with the CSS rules for the ad class you should clean up the act of your theme.
The ad is within the <div class="content">, but should have its own class or id (id="ads") so that you can give it some CSS rules in your stylesheet ( /wp-content/themes/blix/style.css )
By the way - look out for what the ad service at http://www.advertising.no sends you. Hopefully not complete html pages.