Nuxt Ui Guideline: Customization — Use app.config.ts for theme overrides. Description: Runtime theme customization. Do: defineAppConfig with ui key. Don't: nuxt.config for runtime values. Good Example: defineAppConfig({ ui: { button: { defaultVariants: { size: 'sm' } } } }). Bad Example: nuxt.config ui.button.size: 'sm'. Severity: Medium. Docs: https://ui.nuxt.com/docs/getting-started/theme/components.