And by that I mean,
An http server that serves word-like rich formatted text documents and includes a web-based rich formatting text editor to write and edit these documents.
With some searching I found these might be candidates
Ghost
WriteFreely / Write.as
Plume
And maybe the following editors in some kind of standalone mode
TipTap
Editor.js
Quill.js
CKEditor 5
Slate.js
The problem isn’t really CSS, it’s that each theme has its own components with their own rules you have to learn. There’s no real standardization even within one kind of SSG.
It would literally be much easier to just write everything the old fashioned way by hand.