As usual, a video showing it:
The current status is that the infrastructure required for the stylesheet editor is in place:
I figured out how to turn the StyleSheet object back into a JSON stylesheet.
The changes are done and merged back into the same file.
There is UI for 3 of the 6 required pieces. The missing ones are text styles, embedded fonts, and config options (like, break subsections to odd pages)
The UI for the whole app is pretty rough, I am in full make-it-functional mode right now. It will be rethought later on.