KeenWrite, my free and open-source (R) Markdown editor, has a fair amount of overlap with Lunette. From the other thread comments, here are a few items that KeenWrite has:

* Offline editing

* Dark mode

* Manuscript theme

* Separates content from presentation

* Automatically curls straight quotes

Here's a tutorial showing how themes (and variables) work:

https://www.youtube.com/watch?v=3QpX70O5S30

I've been dog-fooding the editor for almost eight years to help me write a sci-fi novel. (Alpha readers wanted!)