Continuing the discussion from Different theme for admin backend?:
The current recommended workaround for not theming the admin UI is:
Discourse supports SCSS which means that you only need to add
body:not(.admin-interface)once to your theme. You don’t need to add it to every rule.
On first look it seems a bit problematic because the :root selector where various colors are defined is above <body> and custom colors still affect both admin and user interfaces.
It would be easier if the <html> tag also had an .admin-interface (or a variant thereof) class. (Or, better still, theme customizations would be even easier if a separate (default) theme could be configured for the admin UI.)
If Discourse enabled theme creators to just theme the parts visible to normal users, it would probably make theme creation/customization easier.
A somewhat related topic is using a separate language for the admin UI (discussed here: Can discourse have different language interfaces for admin only?) — it would be especially helpful for tweaking translations for languages that have very rough (meaning: incorrect) or incomplete coverage (lots of strings untranslated).
I’m currently setting up Discourse in Estonian, and would like to fix bad user-facing translations as I see them, but using the admin UI in Estonian is very confusing because so many texts are incorrect or simply not understandable.