how do i change my headers and footers = /?

hi all,i wonder how i could change my headers and footers,like in other boards,like a proboard,you know....change stuffs like forums width and length and stuffs,global header,main header,main footer global footer usualy exist.
i would like to change my forums width just..thats all,i know the code,but i dont know how to come in the area were i can change headers and footers.
please help me!
//Zizor

CSS? Go to Style -> Pictures & Colors -> Colors, you'll find the link to the style editor