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?