in wpf style, trigger opening son component element attatched

the following rudimentary impression listboxitem, listboxitem son border. border stuffing ability value 8, i wish change value 0, vigilant selected. i trigger?

    <??style targettype="{x:type listboxitem}">
        <setter property="background" value="transparent" />
        <setter property="template">
            <setter.value>
                <controltemplate targettype="{x:type listboxitem}" >
                    <border 
                        snapstodevicepixels="true" 
                        horizontalalignment="center" 
                        verticalalignment="center"
                        stuffing = "8"
                        background="{templatebinding background}">
                        <contentpresenter />
                    </border>
                    <controltemplate.triggers>
                        <trigger property="isselected" value="true">
                            ??<setter property="padding" value="0" />??   <----how i this?
                        </trigger>
                    </controltemplate.triggers>
                </controltemplate>
            </setter.value>
        </setter>
    <??/style>