Sorry, seems as this was posted in the classic selfhosted forum, which is for server issues, that it would be outside our scope of support.
However, it sounds like you may have mod_security or something else security wise if you are receiving this permission error. I would suggest disabling this to test further.
These templates and CSS files are stored in the database so typically wouldn’t be a write issue to the server itself, it would be a processing issue on the server which is being blocked.