stylesheet.xml

<?xml version="1.0" encoding="utf-8"?>
<Map srs="epsg:3857" background-color="steelblue">

    <Style name="style">
        <Rule>
            <PolygonSymbolizer fill="white" clip="false"/>
        </Rule>
    </Style>

    <Layer name="world" srs="epsg:3857">
        <StyleName>style</StyleName>
        <Datasource>
            <Parameter name="file">data/world_merc.shp</Parameter>
            <Parameter name="encoding">iso-8859-1</Parameter>
            <Parameter name="type">shape</Parameter>
        </Datasource>
    </Layer>

</Map>