How to override custom severity icon in a p:confirmDialog

Question

How can I override the severity facet of confirmDialog ?

For example,

<f:facet name="severity">
    <h:outputText rendered="#{severity eq 'alert'}" styleClass="ui-icon icon_warning ui-confirm-dialog-severity"></h:outputText>
    <h:outputText rendered="#{severity eq 'info' }" styleClass="ui-icon icon_info ui-confirm-dialog-severity"></h:outputText>
    <h:outputText rendered="#{severity eq 'warn' }" styleClass="ui-icon icon_error ui-confirm-dialog-severity"></h:outputText>
</f:facet>

Thanks in advance..