extjs4 设置grid header的背景颜色及字体颜色

var mygrid = Ext.create('Ext.grid.Panel', {
			    title: 'Simpsons',
			    store: Ext.data.StoreManager.lookup('simpsonsStore'),
			    columns: columnFiled,
			    height: 200,
			    width: 400,
			    renderTo: Ext.getBody(),
			    listeners : {	
					'afterrender' : function(grid) {
						var elments = Ext.select(".x-column-header",true);//.x-grid3-hd
						elments.each(function(el) {
										el.setStyle("color", 'green');
										el.setStyle("background", '#ff0000');
									}, this);
							
						}
				}
			});