The <themes> element has the following definition:

<!ELEMENT themes (theme+) >

The <themes> element specifies one or more <theme> elements (described in theme Element). If you have specified a base map (basemap attribute of the map_request element), any themes that you specify in a <themes> element are plotted after those defined in the base map. If no base map is specified, only the specified themes are rendered.

Inside this <themes> element there must be one or more <theme> child elements, which are rendered in the order in which they appear.