Error msg when I try to save

Hi, whenever I try to save I get this error message:

Forbidden

You don’t have permission to access /wp-admin/themes.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Any tips?
Thanks-
Michele

https://wordpress.org/plugins/simple-custom-css/