There is a site default in Admin → Forums, and every category can either follow it or pick its own.
🚨 A category set to follow the site is not the same as one set to rows. The first has declined to have an opinion and moves when you change the site default; the second has an opinion and stays. Pick explicitly only where a section is genuinely different from the rest.
Readers can override both from the forum index, and their choice wins. That is the same argument dark mode already won: you decide what the site is, and somebody reading it decides how they read it.
Cards are worth it when the forums inside have cover images. One with no cover still works — it gets a colour derived from its name, so it is the same colour every visit.