Hi Dominik,
This is something I didn’t anticipate when releasing child themes – but after chatting with some other developers, there’s no way around losing child theme modifications if you update the theme.
I’ve added notices in the style.css and functions.php files saying this – so hopefully this doesn’t happen again.
I’m really sorry that you lost your CSS!
From now on, I suggest using a plugin like this: https://wordpress.org/plugins/simple-css/
And any functions you need to add should be added in plugin form.
Of course, if anyone is using their own child theme, it’s safe to use it to add modifications – but the child themes on WordPress.org will be updated periodically, so it’s not safe to edit them.
Sorry again!