Ext的Menu---改变menu引用的CSS

var ctxMenu = new Ext.menu.Menu({...})
//重写createEl 方法
ctxMenu.createEl = this.menuCreateEl;

//改变class名
menuCreateEl: function(){
return new Ext.Layer({
            cls: "x-treemenu",
            shadow:this.shadow,
            constrain: false,
            parentEl: this.parentEl || document.body,
            zindex:15000
        });
},