How can I change the preview (og:image) with the theming app?

When I paste a link into WhatsApp or some other applications like this forum, the banner image (og:image) is set to
This is basically the default Nextcloud icon which looks like this:


How can I use my own banner image? Can I do this within the theming app? If not, how can I do this manually without losing the changes I made in the theming app?

(Env: Nextcloud 15.0.11)

Ok, since nobody replied I thought I try to create a custom theme directory and place my own image there hoping that it wouldn’t destroy my settings in the theming app.

I added the following file themes/mytheme/core/img/favicon-fb.png and set 'theme' => 'mytheme', in config.php.

Yet, it still shows the old image above and not my image.

I emptied all caches to no avail. It seems the config option is totally ignored.

I came to the conclusion that Nextcloud’s theming process does not work. At least not the one using the /themes/ directory.

I therefore replaced the file in /core/img/. Yes, this is bad, but the only way to get it working.

Thank you all for your many replies and help!