How do you change a message boards' layout via html coding in a post?

I've seen it in "epic" message boards, like MFGG (Mario Fan Games Galaxy).





Example: http://img291.imageshack.us/img291/2823/...

How do you change a message boards' layout via html coding in a post?
If you're the administrator of the site (forum), then you should know what files you need to access (generally the CSS). If the forum is not yours, then you can only use the themes provided by the owner.





But you can't do this in a post. You need to access various files of the forum.