Hello, Renate.
The configuration files for a database in Opac are located in the database itself in a directory called opac. See MARC database.
As reported in this Readme (https://github.com/ABCD-DEVCOM/ABCD2/blob/master/www/htdocs/opac/readme.md) The directory OPAC_CONF stores the files of common use the Opac and not more its databases.
To configure it from ABCD, go to ABCD Settings -> OPAC
Best regards,
Roger