Hi @gperna,
we already talked about this in another thread: Nethserver 7 Let’s Encrypt, redirect webgui, plugin responsive roundcube webmail - #4 by gperna
/etc/roundcubemail/config.inc.php
If you change config files do it via the template system:
http://docs.nethserver.org/projects/nethserver-devel/en/v7/templates.html
So you may create /etc/e-smith/templates-custom/etc/roundcubemail/config.inc.php/91MYCHANGES and put your config changes in this file.
$config['skin_logo'] = /usr/share/roundcubemail/mylogo.png;
Then expand the template to write it to the system config files
expand-template /etc/roundcubemail/config.inc.php