I have created an OpenStreetMap server using the guide Manually building a tile server (12.04). It's very good and works like a charm. I have also been able to add a new theme using TileMill and osm-bright.
However, I now want to be able to switch between two themes, osm-bright and my own osm-night. Is this an Apache question or is there a way to modify Mapnik to be able to get a parameter from, for example, openlayers to switch between these two themes?