In config.php, If we change the value of $phpbb_adm_relative_path , the installation system is not able to load the template files.
As on line 119 in /install/startup.php, the admin_path will never be set, the admin_path will always be set with adm/
To fix that point, i suggest to add the following code before the line 119
After adding this code, this will allow to load the template.
But the CSS fails to be loaded, because adm/ is hardcoded in phpbb\install\controller\helper.php on line 124
For this point, I don't know how to fix it.
Thanks in advance for taking this bug into account.
— Edit —
Another instance of adm/ located on line 260 in phpbb/installer/install.php
Maybe adm/ is present in other place.