Wow, great idea! I wouldn't have thought of that but you are absolutely right. Applying individual snippets is excellent. Potentially you could choose from a gallery which styles you want to turn on or off!
I agree that it is possible to achieve quite a bit of functional change through regex that it would be nice enabling some type of cross-theme support without having to modify the json of each theme... I believe Eduard couldn’t see this becoming a full plugin but I guess it could be a a ‘snippet’ as you propose. Alternatively there could be a snippets json file that all themes could reference in the theme file, then a user only needs to reference the same code once and have it supported in All themes. That way there would just be a single snippet file, themes would still need to support it to display the advanced functionality (new user friendly) and snippet users have one location to update snippets that will then be displayed in all themes so that updating multiple theme files is no longer necessary.
Andrew Ford: agree - this would be an easy first step toward a full-fledged implementation. It's too bad that json doesn't have native support for "include" directives. It could be handled by a preprocessor of sorts on the desktop/iCloud side, much like @jgclark's nPTools behaves
I like the idea of choosing which sorts of things I want styles for so that I can turn up or down the regex workload as needed for my uses.