Nuxt Ui Guideline: Theming — Extend semantic colors in nuxt.config. Description: Register new colors like tertiary in theme.colors. Do: theme.colors array in ui config. Don't: Use undefined colors. Good Example: ui: { theme: { colors: ['primary', 'tertiary'] } }. Bad Example: <UButton color="tertiary"> without config. Severity: Medium. Docs: https://ui.nuxt.com/docs/getting-started/theme/design-system.