How to change (sharing-)email subject in a custom theme

Nextcloud version (eg, 11.0.): 11.0
Operating system and version (eg, Ubuntu 16.04): Debian Stretch
Apache or nginx version (eg, Apache 2.4.25): nginx 1.10.2

The issue you are facing:

I do not understand, how to change the subject from emails in a custom theme.

Where can I do this? I can not locate the responsible template file.

It would be fantastic, if you can point me to the correct information (or issue/bug)!

Thank you in advance,