gistfile1.txt

<apex:page showHeader="false" title="New Order" standardController="Service_Order__c">
  <apex:composition template="{!$Site.Template}">
    <apex:define name="body">  

            ...

            <apex:form styleClass="form" >
            <apex:pageMessages id="styledError"/>
            <apex:messages id="unstyledError"/>
            ...

            <apex:commandButton value="Submit" action="{!save}" styleClass="btn btn-default" rerender="error,styledError,unstyledError"/> 
            </apex:form>

        </div>
    </div>

    </apex:define>
  </apex:composition>
</apex:page>