You can use this extension instead of editing styles: http://www.artodia.com/downloads/phpbb31/extranav.zip
Its a basic extension that adds few links to navigation. I'm using it on my dev forum to add extra dummy links to navigation to test responsive navigation.
Unpack it to directory "ext", install.
Then go to directory ext/arty/extranav/styles/all/template/event. In that directory you'll find 5 template files for different navigation items. Each file has comment explaining where that navigation appears. Remove ones you don't need, edit navigation in templates that you need. Most likely you need to keep only overall_header_navigation_prepend.html. Purge cache after editing or deleting templates.