Well, I changed the Selftext font in Cards view, then switched back to Small Cards view, and the setting stuck. Odd, but works.

I tapped your settings link because I was really surprised to see it called "selftext". That's an old reddit term back when they first added posts that are just text. But this is what I see when I tap that, and what gets copied:

Settings shortcut: View type > Description font

I guess it must have changed in an update. It's a good change.

I guess I should have specified that I use the Small Cards view, which doesn't have the Selftext setting.