When you're on the "My Mibba" page there's a link on the right hand colum called "Layout/HTML" click on that and near the bottom/middle there should be something about 'visit the message board for more help', so click on that then open the forum called 'share your layouts'. once you get one, copy the CSS form of it and paste it into the CSS box, which is also a link on your my mibba page called "CSS"
it's pretty easy.