Shadcn Guideline: Theming — Use CSS variables for colors. Description: Define colors as CSS variables in globals.css for theming. Do: CSS variables in :root and .dark. Don't: Hardcoded color values in components. Good Example: bg-primary text-primary-foreground. Bad Example: bg-blue-500 text-white. Severity: High. Docs: https://ui.shadcn.com/docs/theming.