Which CSS-file is the right?

Hey guys!! =)

I've got one question: I want to change the default-Theme (view). I changed the graphics in the folder "_graphics" and changed the attributes in css.php in the folder "views/default/" - But it seems that it doesn't matter changing them, because the website doesn't change optically (except of the new header...). Can you please help me?

Greetz, Minamato