I updated the page structures for the galaxy, userCenter and admin themes. I declared these structure changes to the theme service using the themes extension point in my theme-contrib.xml file:

<require>org.nuxeo.theme.nuxeo.default</require>
<require>org.nuxeo.ecm.user.center.theme</require>
<require>org.nuxeo.ecm.admin.center.theme</require>
<extension target="org.nuxeo.theme.services.ThemeService" point="themes">
  <theme>
    <src>themes/document-management.xml</src>
  </theme>
  <theme>
    <src>themes/user-center.xml</src>
  </theme>
  <theme>
    <src>themes/admin-center.xml</src>
  </theme>
</extension>

I turned on the DEBUG mode in the lib/log4j.xml

<category name="org.nuxeo.theme">
  <priority value="DEBUG" />
</category>

The log/server.log file indicates that the theme is being registered as well as overriding a previous theme…

DEBUG  [org.nuxeo.theme.types.TypeRegistry] Registered theme THEME: themes/document-management.xml
DEBUG  [org.nuxeo.theme.types.TypeRegistry] Overriding theme THEME: themes/document-management.xml
DEBUG  [org.nuxeo.theme.types.TypeRegistry] Registered theme THEME: themes/user-center.xml
DEBUG  [org.nuxeo.theme.types.TypeRegistry] Overriding theme THEME: themes/user-center.xml
DEBUG  [org.nuxeo.theme.types.TypeRegistry] Registered theme THEME: themes/admin-center.xml
DEBUG  [org.nuxeo.theme.types.TypeRegistry] Overriding theme THEME: themes/admin-center.xml

But these page structures do not get overridden unless I rename the xml files to something other than document-management.xml, user-center.xml, admin-center.xml. Only then are these changes reflected in the browser.