function buttonOnmouseover(id, button_type) {
 // change css classes for left, middle and right parts of a button
    document.getElementById(id).className = "button_mouseover";
    document.getElementById(button_type + '_button_left').className = "button_left_mouseover";
    document.getElementById(button_type + '_button_right').className = "button_right_mouseover";
}

function buttonOnmouseout(id, button_type) {
// set default css classes for left, middle and right parts of a button
        document.getElementById(id).className = "button";
        document.getElementById(button_type + '_button_left').className = "button_left";
        document.getElementById(button_type + '_button_right').className = "button_right";
}