did you install the template with quickinstall?

No Dirk, it was added individually.
The bug was confirmed on Astroid Framework (see the Github discussion: github.com/templaza/astroid-framework/issues/219 )

For custom.css file in Astroid 2.6.1 you can use following path
- images/{template}/css
- templates/{template}/css
- media/templates/site/{template}/css

My problem is not the custom.css file but the styles list in a lot of sites with Astroid One, Zero and some of your templates: with JCE that list is empty.
The custom.css in both template or media/templates/sites is well used to personalize the site.

This morning I checked 16 remote sites and I explain the results on the Github discussion.