function mainStyle(nid) {
    var i = 0;
    var nav = "";
    var cont = "";
    var nobj,cobj;
    for (i = 1; i < 7; i++) {
        nav = "nav_" + i;
        cont = "cont_" + i;
        nobj = document.getElementById(nav);
        cobj = document.getElementById(cont);
        if (i == nid) {
            nobj.style.backgroundImage = "url(images/content_menu_bg2.gif)";
            cobj.style.display = "inline";
        }
        else {
            nobj.style.backgroundImage = "none";
            cobj.style.display = "none";
        }
    } 
}