Tweak Theme Overview page to group sub-themes by parent theme

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.