...and after having dealt with it for two years, I can say that it was bad idea to have static HTML fallback for React.js application, as it turned out not only annoyingly time consuming to implement changes to the theme, it is also confusing to people trying to customize theme further
Would it be possible to separate these two things?
Not really. React.js component needs to takeover anything between navbar and footer to work as we are also updating the header when user navigates to other category.
Short term solution would be adding new setting to "general" settings, but adding settings for customizing how things look to the admin panel is something I'm weary of. I've used to have bunch of settings controlling how things were displayed in my first forum software, and that was nightmare to maintain.
In long run I would like to have a way for people to customize Misago easily via "page builder" in our admin panel, but thats years away.