I put a custom css file in [custom.files]/custom/ (wp-content/uploads/civicrm/custom/) and it is not being read. I tested the same test entry in the civicrm/civicrm.css and it works there.
The entry for Resource URLs Custom CSS URL is [civicrm.files]/custom/
I cleared the cache for civicrm and Wordpress and it doesn't help.
I'm using WordPress 5.6 and civicrm 5.32.2.