Google Calendarで過ぎた日の色を変える

(function(){
    var PAST_COLOR = "#e0e0e0";
    var PAST_BORDER_LEFT = "1px solid #d0d0d0";
    var TODAY_COLOR = "#ffa";

    var timer = null;
    function schedule(){
        if(timer){
            clearTimeout(timer);
        }
        timer = setTimeout(update, 100);
    }

    function update(){
        if(document.getElementsByClassName("st-bg-today").length === 0){
            return;
        }

        var bgs = Array.prototype.slice.call(document.getElementsByClassName("st-bg"));
        bgs.find(function(e,i,a){
            if(e.className.match("st-bg-today")){
                e.style.backgroundColor = TODAY_COLOR;
                return true;
            }
            else {
                //e.className += " st-bg-past";
                e.style.backgroundColor = PAST_COLOR;
                e.style.borderLeft = PAST_BORDER_LEFT;
                return false;
            }
        });
        var dtitles = Array.prototype.slice.call(document.getElementsByClassName("st-dtitle"));
        dtitles.find(function(e,i,a){
            if(e.className.match("st-dtitle-today")){
                e.style.backgroundColor = TODAY_COLOR;
                return true;
            }
            else {
                //e.className += " st-dtitle-past";
                e.style.backgroundColor = PAST_COLOR;
                e.style.borderLeft = PAST_BORDER_LEFT;
                return false;
            }
        });
    }

    var observer = new MutationObserver(schedule);
    observer.observe(document.body, {childList:true, subtree:true});

    update();
})();