Since NagVis 1.1x you can add templates to customize the look of header- and hover-menus. You can get some of these templates at the NagVis template download page.
Please note: The directory structure changed between the NagVis version several times. So there may be compatibility problems between the versions.
Unpack the NagVis template in a path of your choice.
tar xvzf NagVis-Template-<name>-<version>.tar.gz
Merge the NagVis-Template directory tree with your NagVis Folder (default /usr/local/nagvis/share/userfiles/templates).
cp -r nagvis /usr/local/nagvis/share/userfiles/templates
Now you should be able to use the new template in NagVis. You only have to configure the maps or objects to use the template.
You can set the header template in the main configuration of NagVis or in the global section of the map configuration. For details see map configuration format description.
You can set the hover template in the main configuration for all your maps, in the global section of a map for all the objects on this map or in each object for having a custom hover menu for special objects. For details see map configuration format description.