Middleweight Themes (after Firefox 57)

There has been talk in various blogs and forums that after
Release 57 there will be a variety of theme which will be
somewhat more configurable than the current Personas
(aka. lightweight themes) and the old style full themes.

Has the planning on this reached the point of having a
tracking bug or a dedicated blog?

I call them Theme API and you find here some ressources about it:
Blog: https://blog.mozilla.org/addons/2017/02/24/improving-themes-in-firefox/
More detailed “documentation”: https://docs.google.com/document/d/1ueD6V7aLLTuc1GAOxxQYcwl2HR-k62HHu3q8knTJ4FU/pub