Tailwind CSS v4 fundamentally changed how theming works. If you're upgrading from v3 or starting fresh, understanding these changes is essential - especially for Nuxt UI projects.

The Big Shift: CSS-First Configuration

Tailwind v3: JavaScript Config

// tailwind.config.js (v3)
module.exports = {
  theme: {
    extend: {
      colors: {
        brand: {
          500: "#6366f1",
          600: "#4f46e5",
        },
      },
      borderRadius: {
        DEFAULT: "0.375rem",
      },
    },
  },
};

Tailwind v4: CSS Config

/* app.css (v4) */
@import "tailwindcss";

@theme {
  --color-brand-500: oklch(0.585 0.233 277.117);
  --color-brand-600: oklch(0.518 0.233 278.117);
  --radius-default: 0.375rem;
}

The @theme directive replaces tailwind.config.js for most use cases. Configuration lives alongside your styles in CSS.

Key Differences

1. Native CSS Variables

Every theme value in v4 becomes a CSS custom property. This means:

• Runtime theming - change values with JavaScript without rebuilding
• Media query adaptation - use @media (prefers-color-scheme: dark) directly
• Component-scoped overrides - override variables at any DOM level

@theme {
  --color-primary: oklch(0.585 0.233 277.117);
}

/* Override for a specific section */
.promo-section {
  --color-primary: oklch(0.7 0.25 330);
}

2. OKLCH Color Space

Tailwind v4 uses OKLCH instead of HSL or hex. OKLCH provides:

• Perceptual uniformity - equal lightness steps look equally bright
• Wider gamut - access more vivid colors on modern displays
• Better dark mode - adjusting lightness produces natural results

/* OKLCH format: oklch(lightness chroma hue) */
--color-primary-500: oklch(0.585 0.233 277.117);
--color-primary-600: oklch(0.518 0.233 278.117);

3. No More tailwind.config.js (For Most Cases)

The JavaScript config file is optional in v4. Basic theming, colors, fonts, and spacing are all handled in CSS:

@import "tailwindcss";

@theme {
  --font-sans: "Inter", ui-sans-serif, system-ui, sans-serif;
  --font-mono: "JetBrains Mono", ui-monospace, monospace;
  --color-primary-500: oklch(0.585 0.233 277.117);
  --radius-lg: 0.5rem;
}

4. Simplified Dark Mode

With CSS variables, dark mode is just another variable assignment:

@theme {
  --color-bg: white;
  --color-text: oklch(0.15 0 0);
}

@media (prefers-color-scheme: dark) {
  :root {
    --color-bg: oklch(0.15 0 0);
    --color-text: oklch(0.9 0 0);
  }
}

5. New Color Syntax in Utilities

Tailwind v4 introduces a new syntax for referencing CSS variables in utility classes:

<!-- v4 variable reference syntax -->
<div class="bg-(--color-primary-500) text-(--color-text)">Content</div>

What This Means for Nuxt UI

Nuxt UI v4 fully embraces these changes:

• Semantic color roles (primary, secondary, etc.) are mapped to CSS variables
• Theme configuration in app.config.ts sets palette names, and Nuxt UI generates the CSS variables
• The Theme Builder outputs CSS variable overrides compatible with Tailwind v4's system

Your workflow:

1. Define color roles in app.config.ts (palette names)
2. Override specific shades with CSS variables if needed
3. Tailwind v4 utilities automatically use the correct values

Migration Checklist

If upgrading from Tailwind v3:

• Replace tailwind.config.js color definitions with @theme in CSS
• Convert hex/HSL colors to OKLCH
• Move custom fonts from JS config to CSS @theme
• Update custom utility references to use (--var) syntax where needed
• Test dark mode with the new variable-based approach

Try the New System

The Nuxt UI Theme Builder generates Tailwind v4-compatible output. Configure your theme visually, then export as CSS variables that slot directly into your @theme block.

Next Steps

• CSS Variables Reference for Nuxt UI Themes - complete variable list
• How to Customize Nuxt UI Theme Colors - configure colors the Nuxt UI way
• Nuxt UI Component Styling Cheat Sheet - style individual components