Mail logo not working with custom theme


I have one custom theme for my company and have realized that the logo-mail.gif is not used when the system sends mails to users: the logo used is the NC defaults.

If I try with the example theme the behavior is the same.

How can I change the logo so that NC can use the one I have at my theme dir?

Nextcloud version : 17.0.0
Operating system and version: Ubuntu 18.04
Apache or nginx version: 2.4.29
PHP version: 7.2.24