I'm not entirely certain what you mean when you say you're using wget to "grab the content" you want but not getting the .css files.
Is this your own site, presumably? Or one on which you have permission? Are you saying there are definitely .css files that wget isn't picking up?
I'm a little confused by your question.
If you use the code:
you should not only get the webpage but also all other content on the server that goes with it.
Hope this helps.