It would be nice to have a first-party solution for sharing common CSS between all of the cards in a deck. There is a workaround (chrisk91(dot)me/2017/12/20/Sharing-CSS-across-multiple-cards-in-Anki.html) but an in-app solution would be simpler from the average user’s perspective.
I think the add-on asset manager allows you to do that.
Asset manager, is also per-notetype, not across all notetypes.
Maybe you can put your style contents into a local CSS file in
collection.media, then import it in Styling.