IBM Plex Sans is the primary face. Display, body, and condensed roles all resolve to it by default. Use it for documentation, dashboards, settings, forms, marketing pages, and any prose-heavy or proportional-label-heavy surface.
--font-mono is always IBM Plex Mono — code blocks, terminal output, data values, and timestamps stay monospaced regardless of mode.
Apply .mono-mode at the surface root. Children inherit. Display, body, and condensed all flip to Plex Mono.
One mode per application (or per major surface). Do not mix Sans and Mono body text on the same screen — the rhythm clash breaks the system. Mono for code, data, status badges, and timestamps is always allowed and expected, even in Sans surfaces.