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Tailwind CSS v4 in Next.js 14: What Changed and How to Migrate

#webdev #nextjs #tailwindcss #css

Tailwind CSS v4 in Next.js 14: Migration Guide and New Features

Tailwind CSS v4 shipped with a completely overhauled configuration system. If you're starting a new Next.js project or upgrading, here's what changed and how to get set up correctly.

What Changed in v4

No more tailwind.config.js. Configuration moves into your CSS file using @theme.

Native CSS variables. All design tokens are CSS custom properties by default.

Faster builds. The new Rust-based engine is 10x faster than v3 for large projects.

New import syntax. Instead of @tailwind base/components/utilities, use @import 'tailwindcss'.

New Project Setup (v4) 

npx create-next-app@latest my-app --tailwind
# Tailwind v4 is included by default in recent create-next-app
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If it installs v3, upgrade:

npm install tailwindcss@next @tailwindcss/postcss@next
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The New CSS File Format

v3 globals.css:

@tailwind base;
@tailwind components;
@tailwind utilities;
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v4 globals.css:

@import 'tailwindcss';

@theme {
  --color-brand: #c8102e;
  --color-brand-blue: #0062b8;
  --font-sans: 'Inter', sans-serif;
  --radius: 0.5rem;
}
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The @theme block replaces tailwind.config.js theme.extend. All values become CSS custom properties automatically.

Dark Mode

v3: darkMode: 'class' in config.

v4: Add to CSS:

@import 'tailwindcss';

@variant dark (&:where(.dark, .dark *));
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Usage is identical: dark:bg-gray-900 dark:text-white

Custom Colors

v3 (config):

theme: {
  extend: {
    colors: {
      brand: '#c8102e',
    },
  },
}
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v4 (CSS):

@theme {
  --color-brand: #c8102e;
  --color-brand-50: #fef2f4;
  --color-brand-900: #4a030d;
}
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Usage: bg-brand, bg-brand-50, text-brand-900 — same as before.

Custom Fonts 

@import 'tailwindcss';
@import url('https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&display=swap');

@theme {
  --font-sans: 'Inter', ui-sans-serif, system-ui, sans-serif;
  --font-mono: 'JetBrains Mono', ui-monospace, monospace;
}
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Migrating from v3

The Tailwind team provides an upgrade tool:

npx @tailwindcss/upgrade
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What it handles automatically:

  • Converts tailwind.config.js theme values to @theme CSS
  • Updates the import syntax
  • Migrates deprecated utility names

What to check manually:

  • Custom plugins (plugin API changed significantly)
  • safelist patterns (now handled differently)
  • PostCSS config (now uses @tailwindcss/postcss instead of tailwindcss)

PostCSS Config

v4 postcss.config.mjs:

export default {
  plugins: {
    '@tailwindcss/postcss': {},
  },
};
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shadcn/ui Compatibility

shadcn/ui supports Tailwind v4 as of the canary channel. When initializing on a v4 project:

npx shadcn@canary init
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Components are generated with v4-compatible class names.

Pre-Configured in the AI SaaS Starter Kit

The AI SaaS Starter Kit ships with Tailwind v4 configured — custom colors, dark mode, shadcn/ui integration, and the PostCSS setup.

AI SaaS Starter Kit — $99

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