Shadcn Guideline: Theming — Support dark mode. Description: Include .dark class styles for all custom CSS. Do: Define both :root and .dark color schemes. Don't: Only light mode colors. Good Example: .dark { --background: 240 10% 3.9%; }. Bad Example: No .dark class styles. Severity: High. Docs: https://ui.shadcn.com/docs/dark-mode.