Replace hardcoded dark-palette Tailwind classes with semantic CSS custom properties (page/surface/raised/line/ink/accent/sev-moderate/sev-major/ map-*) defined on :root (light) and .dark (dark), wired through tailwind.config.ts theme.extend.colors. Add IBM Plex Sans/Mono via next/font/google: sans for everything a person reads, mono reserved for machine identifiers only. Drop font-mono from body and Button's base classes. Existing !important light-mode overrides kept temporarily so nothing goes unreadable mid-migration (removed in chunk 4). Per UI_REDESIGN.md chunk 1.