Cannot use custom themes. Editing the file Z.php doesn't makes custom themes available.

Although, editing the function 'getThemes()' in ZBase.php makes the themes available.

The file 'Z.php' doen't seem to be part of the frontend anymore in CentOS 8:

# rpm -qf /usr/share/zabbix/include/classes/core/Z.phpfile /usr/share/zabbix/include/classes/core/Z.php is not owned by any package

Steps to reproduce:

1. Follow the instructions form section "18.8 -  Creating your own theme" of the manual
2. Navigate to screen "User settings" / "User profile"
3. Click on screen element "Themes", the custom theme doesn't show up.

Result:

Expected: