How to apply CSS to the content ?

I could not find it in the documentation, but how to apply a CSS on the 'what you see is what you get' content ? The styles do not have to show up in the styles drop down (so conversion to xml is not where I'm looking for), but I want to have a real WYSISYG result... which I do not have now. I use external stylesheet. If filex uses stylesheetx, filey uses stylesheety I want to make it 'visible'... how ????