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:

craft_cms-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 Link: 
https://github.com/backdrop/backdrop-issues/issues/986
GitHub Issue #: 
986