gistfile1.xml

<!-- in theme042/layout/catalog.xml -->

<reference name="root"> <!-- means that for this page we're going to update the root block (aka the whole page)

    <!-- this magick incantation changes the template for this page. Paths are relative to 
          theme042/template, and if they don't exist there, they're searched for in 
          default/template -->
    <action method="setTemplate"><template>page/2columns-right.phtml</template></action>


</reference> <!-- always ensure your docs are well-formed -->