Changing aggregator appearance

I would like to use feeds obtained by the core aggregator module to display more or less the same as nodes, thus with rounded edges (see www.covs.org). I tried to apply the same trick mentioned in http://drupal.org/node/324690#comment-1080099, but this does not work. Does this have to do with aggregator not producing nodes?

Could anyone help me with this or give me a pointer where to look?

Thanks in advance!