This may also have to do with upgrading to Sierra and the latest Storyist, but the little red arrow and drop down menu for editing the various style sheets isn't visible on the Novel template sidebar. When I try and enlarge the page, the sidebar remains the same size so I can't stretch the page and enlarge the sidebar. I can barely see a little bit of the boxes to the right of all the spacing selections and tab section for editing those.
Since I was looking for them, I can access the edit window which drops down in the middle of the page from the top if I click at the edge of the page by header, or section text, etc.
If it's a Sierra problem, a new user running Sierra wouldn't realize they can edit or create new style sheets via this route.