Child Theme: 400 - Template "default.html.twig" is not defined

Hi,

I want to make a child theme with quark-open-publishing as parent theme. So I read this article https://getgrav.org/blog/theme-development-with-inheritance

My mytheme.yaml looks like this

streams:
 schemes:
   theme:
 type: ReadOnlyStream
 prefixes:
   '':
     - user/themes/mytheme
     - user/themes/quark-open-publishing

Folder structure is:

mytheme in the backend looks like this:

When I active it I get following error message:

Server Error
Sorry, something went terribly wrong!

400 - Template "default.html.twig" is not defined.

When I activate quark-open-publishing then everything is okay. So I guess something is wrong with my child-theme setup

Can you help me with it?