With the discussion in #131 which brought up a point about sub-themes, I decided it was a good time to point out that parent and sub-themes have never had a good UX feeling because the UI was never designed to incorporate them.
Proposed solution
What I propose is to slightly re-design the theme overview page and move the title of each theme into a "group" heading to allow a theme with sub-themes to all be housed together.
Mockup Example:

Draft of feature description for Press Release (1 paragraph at most)
The Backdrop theme overview page has been tweaked to group sub-themes together with their parent themes to create a better UI experience.
Recent comments
@yorkshirepudding, In the end I fixed it by deleting the config/active folder contents, copying the config/staging/ files into config/active, importing the database again and clearing the caches...
Importing the staged config throws an error: another field with the same name already exists
Thanks, I have to go out now, I will try later on and report back
Importing the staged config throws an error: another field with the same name already exists
Hi Francis. Yes, what config do you have in staging for it? You might have configs like: field.field.field_video_embed field.instance.node.page....
Importing the staged config throws an error: another field with the same name already exists