HTML Export delivers broken CSS

We rely on the HTML Export function to deliver some Confluence pages in which we track the status of certain projects to our customers without having to grant them access to our Confluence. We recently upgraded Confluence from 4.3.6 to 5.1.1 and found out that the exported pages don't look like the original versions at all anymore. The exports from 4.3.6 were almost exact copies of the wiki pages, without the header, which was great.

I've tracked the problem down to wrong CSS: if I download the CSS that Confluence uses itself and insert it into the exports, everything looks fine.

I therefore conclude that Confluence exports the wrong CSS when exporting to HTML.