chage wfs layer style

Question

i have a wfs point layer and call it with openlayers. its default css is

fill:#ee9900;
fill-opacity: 0.4;
stroke: #ee9900;
stroke-opacity: 1;
stroke-width: 1px;
stroke-linecap: round;
stroke-linejoin: round;
stroke-dasharray: none;
pointer-events: visiblepainted;
cursor: inherit;

and i want to change it and write some code but i cant achive, my code is below, pls correct me

    var xxx= new OpenLayers.Layer.Vector("xxx", {
        protocol: new OpenLayers.Protocol.WFS({
            url: "http://localhost:8080/geoserver/wfs",
            featureType: "xxx",
            geometryName: "geom",
            featureNS: "http://localhost:8080/SBB",
            styleMap: new OpenLayers.StyleMap({
            'default': new OpenLayers.Style({
                strokeOpacity: 0.5,
                strokeColor: "#88aaaa",
                fillColor: "#99CC55",
                fillOpacity: 0.5,
                fontColor: "#ffffff"
            })
        })
        }),
        strategies: [new OpenLayers.Strategy.BBOX()]
    });