When installing Nuvola style in latest Nagios on Debian Wheezy, there is a problem with stylesheets.
To fix it before copy all content of Nuvola archive you must create in directory
/usr/share/nagios3/htdocs
new symbolic link to /etc/nagios3/stylesheets:
ln -s /etc/nagios3/stylesheets /usr/share/nagios3/htdocs/stylesheets
This way when copy Nuvola files, stylesheet files will go to proper place (/etc/nagios3/stylesheets).