I have the impression that site builders/admins use a different theme for the admin backend. How cool would it be to be able to preview the changes made to the layout/blocks of the frontend theme as they make changes in the UI?
Here's how Craft CMS does it with their WYSIWYG editor:

We already have a live preview in place for views and another for the color-picker, so perhaps we can abstract and reuse that.
GitHub Issue #:
986
Recent comments
Hi Kevin I am interested assisting you developing a theme by cloning feature from existing WordPress website. Please let me know your suitable time to discuss further...
Create a theme from existing website
I've updated the Zulip link in both places I found it. No need to post again, unless you have something new to say. We'll pull together feedback from all the sources.
Backdrop CMS Core Priorities
Should we post here again, what we posted over there? Or would that unnecessarily duplicate things? The link to a Zulip thread in this initial post leads to an internal one, but there's...
Backdrop CMS Core Priorities