Set up Tailwind v4 with shadcn/ui using @theme inline pattern and CSS variable architecture. Four-step mandatory pattern: define CSS variables at root, map to Tailwind utilities, apply base styles, get automatic dark mode. Use when: initializing React projects with Tailwind v4, setting up shadcn/ui dark mode, or fixing colors not working, theme not applying, CSS variables broken, tw-animate-css errors, or migrating from v3.