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
Hi @opentype. Thanks for confirming it worked. It is helpful for others if you can also mark the answer using the "accept this answer" button as this will show that the question has an answer...
Page navigation also above the page content?
Thanks for the detailed instructions. It worked.
Page navigation also above the page content?
Hello opentype. Welcome to Backdrop CMS. This would either require a sub-theme (have a look at Thesis which is a starter sub-theme with nothing changed) or a custom module. Assuming...
Page navigation also above the page content?