Tailwind CSS v4: where did tailwind.config.js go?

Tailwind v4 is configured in CSS. Instead of tailwind.config.js, add your theme to your main CSS file:

@import "tailwindcss";

@theme {
  --color-brand-500: #2563eb;
  --font-sans: 'Inter', sans-serif;
}

Classes like bg-brand-500 then work straight away. Run npm run build after changes so production gets the new CSS.