function centerBox(idBox) {
    var box = document.getElementById(idBox);
    var boxWidth = box.clientWidth;
    var boxHeight = box.clientHeight;

    var leftPos = (posRight()/2)-(boxWidth/2);
    
    
    box.style.left = leftPos+"px";
    
    var topPos = (posBottom()/2) - (boxHeight/2); 
    box.style.top = topPos+"px";
}