atop configuration file location on CentOS 7.x
Solution 1:
In centos 7x, see
/etc/sysconfig/atop
Solution 2:
according to the man page, in the FILES section:
/etc/atoprc
Configuration file containing system-wide default values.
So I guess you should look there. You also have a ~/.atoprc file with your personal preferences.
Maybe interesting to note, the /etc/atoprc file is not automatically created. If you install atop from EPEL repo, it does not add the atoprc manpage either.