This is indeed not the right approach. Your template has to be XML well formed. I suggest to create a master template file instead if all you want is to only specify the center unit.
Taking the example on the showcase site, that should look like this:
/WEB-INF/templates/layout.xhtml
<!DOCTYPE html>
<html lang="en"
     xmlns="http://www.w3.org/1999/xhtml"
     xmlns:f="http://java.sun.com/jsf/core"
     xmlns:h="http://java.sun.com/jsf/html"
     xmlns:ui="http://java.sun.com/jsf/facelets"
     xmlns:p="http://primefaces.org/ui"
>
    <h:head>
        <title>Title</title>
    </h:head>
    <h:body>
        <p:layout fullPage="true">
            <p:layoutUnit position="north" size="100" header="Top" resizable="true" closable="true" collapsible="true">
                <h:outputText value="Top unit content." />
            </p:layoutUnit>
            <p:layoutUnit position="south" size="100" header="Bottom" resizable="true" closable="true" collapsible="true">
                <h:outputText value="South unit content." />
            </p:layoutUnit>
            <p:layoutUnit position="west" size="200" header="Left" resizable="true" closable="true" collapsible="true">
                <h:form>
                    <ui:include src="../templates/themeMenu.xhtml" />
                </h:form>
            </p:layoutUnit>
            <p:layoutUnit position="east" size="200" header="Right" resizable="true" closable="true" collapsible="true" effect="drop">
                <h:outputText value="Right unit content." />
            </p:layoutUnit>
            <p:layoutUnit position="center">
                <ui:insert name="content">Put default content here, if any.</ui:insert>
            </p:layoutUnit>
        </p:layout>
    </h:body>
</html>
Note the <ui:insert> in the center unit.
The template client can then just look like this:
/page.xhtml
<ui:composition template="/WEB-INF/templates/layout.xhtml"
     xmlns="http://www.w3.org/1999/xhtml"
     xmlns:f="http://java.sun.com/jsf/core"
     xmlns:h="http://java.sun.com/jsf/html"
     xmlns:ui="http://java.sun.com/jsf/facelets"
     xmlns:p="http://primefaces.org/ui"
>
     <ui:define name="content">
         <p>Here goes your view-specific content.</p>
     </ui:define>
</ui:composition>
which you open by http://example.com/contextname/page.xhtml.
See also:
If you're looking for live open source examples of advanced Facelets templating, you may find OmniFaces showcase app useful.