Nuxt Ui Guideline: Theming — Configure colors in app.config.ts. Description: Runtime color configuration without restart. Do: ui.colors.primary in app.config.ts. Don't: Hardcoded colors in components. Good Example: defineAppConfig({ ui: { colors: { primary: 'blue' } } }). Bad Example: <UButton class="bg-blue-500">. Severity: High. Docs: https://ui.nuxt.com/docs/getting-started/theme/design-system.