The default installation of Nagios is pretty ugly but can be tarted up with the Nuvola icons and some stylesheets from the following page:
http://www.nagiosexchange.org/cgi-bin/page.cgi?g=Detailed%2F1723.html;d=1
Scroll down the page to the files section and download the Nuvola file. Extract it and copy the contents of the ./html directory into the nagios web directory (/usr/local/nagios/share/html on debian).
Quick and easy!
