You can find the file in static/index.html in your Prodigy installation. To find the path, you can run the following command:
python -c "import prodigy; print(prodigy.__file__)"
If you check out the index.html markup, you’ll see that it’s pretty straightforward and compact. <div id="root"></div> is where the app will be rendered – everything else stays intact. So if you add a <style></style> in the <head>, it will be available when you run the app.
The main downside here (and what a "global_css" setting would solve) is that you have to edit the static source and can’t easily add recipe-specific markup. So if one recipe needs some custom formatting and the other one doesn’t, that’s currently a bit tricky.