Edit /etc/roundcube/config.inc.php and add 'message_highlight' to the
$config['plugins'] array in order to enable this plugin in Roundcube
Webmail.

The plugin can be configured in /etc/roundcube/plugins/message_highlight/config.inc.php .
See /usr/share/roundcube/plugins/message_highlight/config.inc.php-dist
for known configuration options and values.