Really depends. http://answers.google.com/answers/threadview?id=17178 should provide you with some insite as to how (or not) to address your issue. If the page you are calling is from another domain all together I would forget it. Just me :)
It is from another domain as its a 3rd party forum.
The add post and reply to post are put into iframe and we don't have access to the code. All i am doing is calling my CSS at the bottom of the page so that it overrides the forum CSS.
It overrides all the CSS except for the ones in the iframe. :(