Formatting Postie’s posts

I would like to change the way postie’s posts look, i want the video (a 3gp clip, i could do this through extreme video plugin) to show in a player, and i want to add a css style (one like this https://www.alistapart.com/articles/cssdropshadows/) to the photos
how can i do this? is it as easy and changeing some code to include a certin css class or is it somthing more complicated