If you want to create your own navmap 'look and feel' (for example, one with rounded corners), you need to add a customized navmap macro theme file to the WEB-INF/classes/com/atlassian/confluence/plugins
/macros/advanced directory. The file name convention to use is navmap-mytheme.vm. Use the name of your choice for the mytheme part of the file name, which is also the value you use for this parameter. Hence, if your theme was called navmap-roundededges.vm, use the value of roundededges for this parameter.
The theme must be written in Velocity. See Velocity User Guide for more information.