A maintainable theme uses semantic tokens rather than scattering arbitrary hex values across components. Think in roles—surface, surface-raised, text-primary, text-muted, border-subtle, brand, and danger—then map each role to light and dark values.

Check text contrast, disabled controls, form errors, charts, and focus rings in both themes. A muted color that looks elegant on a white background may become unreadable on a dark surface. Test real states, not just the default card.