<objective> Documents Tailwind CSS v4.3's CSS-first configuration directives: `@import "tailwindcss"`, `@theme` for defining theme tokens as CSS custom properties, `@source` for scanning additional source files, `@utility`/`@variant` for custom utilities and variants, `@apply` for composing utilities in custom CSS, and `@config` for loading a legacy JS config.

Also covers dark-mode variant setup, responsive breakpoints via @theme, CSS layer hierarchy (theme, base, components, utilities), and plugin integration — the full CSS-first replacement for tailwind.config.js. </objective>

Tailwind CSS Core v4.3

Overview

Tailwind CSS v4 (since v4.0; current stable v4.3) uses a CSS-first approach that eliminates the need for a traditional tailwind.config.js file. All configuration is now done directly in your CSS files via specialized directives.

Key Concepts

1. @import "tailwindcss"

Entry point to load Tailwind CSS. Place at the beginning of your main CSS file.

@import "tailwindcss";

This directive automatically loads:

Base utilities
Responsive variants
Layers (theme, base, components, utilities)

2. @theme

Directive to define or customize theme values via CSS custom properties.

@theme {
  --color-primary: #3b82f6;
  --color-secondary: #ef4444;
  --spacing-custom: 2.5rem;
  --radius-lg: 1rem;
}

Variables are accessible in generated utilities:

--color-* → classes bg-primary, text-primary, etc.
--spacing-* → classes p-custom, m-custom, etc.
--radius-* → classes rounded-lg, etc.

3. @source

Directive to include additional source files with glob patterns.

@source "./routes/**/*.{ts,tsx}";
@source "./components/**/*.{tsx,jsx}";
@source "../packages/ui/src/**/*.{ts,tsx}";

Tailwind will scan these files to generate utilities used in your project.

4. @utility and @variant

Directives to create custom utilities and variants.

@utility truncate {
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
}

/* Define a new variant with @custom-variant */
@custom-variant theme-midnight (&:where([data-theme="midnight"] *));

/* Apply an existing variant inside CSS with @variant
   (stacked `hover:focus` and compound `hover, focus` forms since v4.3) */
.card {
  @variant hover:focus {
    background: var(--color-sky-600);
  }
}

5. @apply

Directive to apply Tailwind classes in your custom CSS rules.

.btn {
  @apply px-4 py-2 rounded-lg font-semibold;
}

.btn-primary {
  @apply bg-blue-500 text-white hover:bg-blue-600;
}

6. @config

Directive to load external configuration if needed.

@config "./tailwind.config.js";

(Optional in v4, mainly used for backward compatibility)

Dark Mode

Dark mode configuration in Tailwind v4.3:

@import "tailwindcss";

/* Default: `dark:` follows prefers-color-scheme — no config needed */

Or via manual class (override the dark variant with @custom-variant):

@custom-variant dark (&:where(.dark, .dark *));

Responsive Breakpoints

Breakpoints are defined via @theme:

@theme {
  --breakpoint-sm: 640px;
  --breakpoint-md: 768px;
  --breakpoint-lg: 1024px;
  --breakpoint-xl: 1280px;
  --breakpoint-2xl: 1536px;
}

Responsive variants are used with utilities:

<div class="text-sm md:text-base lg:text-lg"></div>

Layer Hierarchy

@layer theme, base, components, utilities;

@import "tailwindcss";

/* Your customizations */
@layer components {
  .btn { @apply px-4 py-2 rounded; }
}

@layer utilities {
  .text-shadow { text-shadow: 0 2px 4px rgba(0,0,0,0.1); }
}

Plugin Integration

Load Tailwind plugins:

@import "tailwindcss";
@plugin "flowbite/plugin";
@source "../node_modules/flowbite";

Specificity Order

In CSS-first, import and declaration order determines specificity:

@import "tailwindcss" - Base utilities
@theme { ... } - Theme variables
@layer components { ... } - Custom components
@layer utilities { ... } - Custom utilities

CSS-first Mode Benefits

No complex JavaScript config file
Type-safe via CSS variables
Declarative and readable configuration
Replaces CSS preprocessors (Tailwind v4 is not designed to be used with Sass/Less/Stylus)
Simplified maintenance for large projects

Detailed References

See specific files for:

theme.md - Complete theme variable configuration
directives.md - Syntax and examples of all directives
config.md - Advanced configuration and use cases