I am using OJS 3.2.1-1 on Linux server with PHP 7.2. When I try to upload a custom css the site confirm the operation, but css is not used.
Debugging the code I found that the problem is in setMode function in
ojs-3.2.1-1\lib\pkp\classes\file\FileManager.inc.php in “return chmod($path, $mask & ~$umask);”.
I don’t know why, but the command is not working and return false. Folders have 777 permission.
Once I had similar issue and the root cause were comments and empty lines in the CCS. After removing them the CCS worked. Don’t ask me how and why, you can give a try and see if it helps to you too.