Use when adding, debugging, or migrating dark-mode support in a Tailwind project: choosing between the default `prefers-color-scheme` strategy and user-toggleable class or attribute strategies, wiring the JavaScript toggle, swapping CSS-variable token values for light vs dark, or porting v3 `darkMode` configuration to the v4 `@custom-variant dark` directive. Prevents the four canonical dark-mode failures: assuming `dark:` utilities need configuration before they work, leaving v3's `darkMode: 'class'` config in a v4 project (silently ignored), forgetting the v4 `@custom-variant dark` directive when migrating from v3 `darkMode: 'class'`, and shipping a toggle that flashes the wrong theme on first paint because the script runs after CSS. Covers: default `media` strategy, class-based toggling (v3 `darkMode: 'class'` and 'selector', v4 `@custom-variant dark (&:where(.dark, .dark *))`), attribute-based toggling (`[data-theme="dark"]`), custom variant strategies, theme-token swap via CSS variables, and the canonical
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Use when adding, debugging, or migrating dark-mode support in a Tailwind project: choosing between the default `prefers-color-scheme` strategy and user-toggleable class or attribute strategies, wiring the JavaScript toggle, swapping CSS-variable token values for light vs dark, or porting v3 `darkMode` configuration to the v4 `@custom-variant dark` directive. Prevents the four canonical dark-mode failures: assuming `dark:` utilities need configuration before they work, leaving v3's `darkMode: 'class'` config in a v4 project (silently ignored), forgetting the v4 `@custom-variant dark` directive when migrating from v3 `darkMode: 'class'`, and shipping a toggle that flashes the wrong theme on first paint because the script runs after CSS. Covers: default `media` strategy, class-based toggling (v3 `darkMode: 'class'` and 'selector', v4 `@custom-variant dark (&:where(.dark, .dark *))`), attribute-based toggling (`[data-theme="dark"]`), custom variant strategies, theme-token swap via CSS variables, and the canonical JavaScript toggle snippet using `localStorage` plus `matchMedia`. Always shows v3 and v4 in side-by-side blocks; never fuses a single snippet that hides the version difference (per D-008). Keywords: dark mode, dark variant, dark:bg-, dark:text-, prefers-color-scheme, darkMode class, darkMode selector, custom-variant dark, where dark class, data-theme dark, toggle dark mode, localStorage theme, matchMedia, next-themes, light dark system, color scheme, how do I add dark mode, dark mode not working, dark utilities ignored, flash of wrong theme, FOUC, flash of unstyled content, OS dark mode, system theme, v3 to v4 dark mode migration, darkMode config removed.
license
MIT
compatibility
Designed for Claude Code. Requires Tailwind CSS v3.4 or v4.0+.
metadata
{"author":"OpenAEC-Foundation","version":"1.0"}
Tailwind CSS : Dark Mode
Three strategies, four moving parts. ALWAYS pick a strategy first, then implement all four parts (CSS configuration, markup, toggle script, optional theme-token swap) consistently. NEVER mix v3 config syntax with v4 directive syntax in one project.
Quick Reference
The three strategies
Strategy
When the dark: variant applies
Pick when
media (default)
OS-level prefers-color-scheme: dark
NEVER need a manual user toggle; ALWAYS respect OS preference verbatim
class / selector
A class on <html> (default .dark)
User has a UI toggle; want to override OS preference
attribute (custom)
A data attribute (e.g., data-theme="dark")
Existing design system already uses attribute-based theming
Three invariants
ALWAYS apply dark: utilities to the element that should react. NEVER attempt a global stylesheet swap; Tailwind's variant model is per-utility.
ALWAYS run the JS toggle BEFORE the page paints. NEVER place the toggle inside a deferred <script> or wait for React hydration; a flash-of-wrong-theme is the visible failure.
ALWAYS keep the strategy choice consistent across v3 and v4 boundaries. NEVER leave a v3 darkMode: 'class' config in a v4 project: v4 silently ignores it.
v3 vs v4 configuration (side-by-side)
The strategy is the same conceptually; only the configuration mechanism differs.
Strategy
v3 (in tailwind.config.js)
v4 (in main CSS file)
media (default)
Omit or darkMode: 'media'
Omit (default)
Class on <html>
darkMode: 'class' (legacy) or darkMode: 'selector' (v3.4.1+)
@custom-variant dark (&:where(.dark, .dark *));
Custom data attribute
darkMode: ['selector', '[data-theme="dark"]']
@custom-variant dark (&:where([data-theme=dark], [data-theme=dark] *));
Fully custom selector
darkMode: ['variant', '&:not(.light *)']
@custom-variant dark (&:not(.light *));
ALWAYS read the table top-to-bottom : pick a strategy first, then apply the v3 column OR the v4 column, never mix.
Decision Tree 1 : Which strategy?
Q1. Does the project need a user-facing toggle (light / dark / system)?
no → ALWAYS choose `media` (default). Done. The `dark:` variant
already works against OS preference; no config needed.
yes → Q2
Q2. Does the existing design system already use a data attribute
(e.g., shadcn / Radix Themes / next-themes default)?
no → ALWAYS use class strategy : v3 `darkMode: 'selector'`
(or legacy 'class'), v4 `@custom-variant dark (&:where(.dark, .dark *))`.
The user toggles `<html class="dark">`.
yes → ALWAYS use attribute strategy : v3
`darkMode: ['selector', '[data-theme="dark"]']`, v4
`@custom-variant dark (&:where([data-theme=dark], [data-theme=dark] *))`.
The user toggles `<html data-theme="dark">`.
Decision Tree 2 : Where do dark styles LIVE?
Q1. Does the same DOM element have light AND dark visual rules?
yes → ALWAYS use `dark:` variant on the utility :
`class="bg-white text-slate-900 dark:bg-slate-900 dark:text-white"`
no → Q2
Q2. Do you have a design system with semantic tokens
(e.g., `--background`, `--foreground`) that should swap?
yes → ALWAYS swap the CSS variable VALUES inside a `.dark` selector
(or `[data-theme=dark]`). See pattern 4. Then markup stays
`class="bg-background text-foreground"` with no `dark:`
variants needed.
no → write `dark:` variants per utility
Decision Tree 3 : Toggle script placement
Q1. Does the framework support inline script in `<head>` (Next.js, Astro,
Vite + index.html, Remix, plain HTML)?
yes → ALWAYS place the toggle as an inline script in `<head>`
BEFORE any CSS link. This runs synchronously before paint,
eliminating the flash.
no → Q2
Q2. Does the framework have an SSR escape hatch (Next.js Script
`beforeInteractive`, Astro fragment, SvelteKit `+layout.svelte`
with `data-sveltekit-preload-data`)?
yes → ALWAYS use it.
no → Use `next-themes` or equivalent that solves the
flash via the same inline-script trick under the hood.
NEVER place the toggle inside a deferred React useEffect, Vue mounted(), or Svelte onMount : it runs after the first paint and visibly flashes the wrong theme.
Patterns
Pattern 1 : Default media strategy
ALWAYS use this when no user toggle is required.
v3 (tailwind.config.js) :
/** @type {import('tailwindcss').Config} */module.exports = {
content: ['./src/**/*.{html,js,ts,jsx,tsx}'],
// darkMode defaults to 'media' ; can also be explicit :// darkMode: 'media',
}
v4 (app/globals.css or equivalent) :
@import"tailwindcss";
/* No darkMode config needed ; `dark:` variants respect prefers-color-scheme by default */
Markup :
<divclass="bg-white text-slate-900 dark:bg-slate-900 dark:text-white">
Reacts to OS preference automatically.
</div>
Pattern 2 : Class strategy (user toggle on <html class="dark">)
v3 (recommended selector form since v3.4.1) :
/** @type {import('tailwindcss').Config} */module.exports = {
content: ['./src/**/*.{html,js,ts,jsx,tsx}'],
darkMode: 'selector', // v3.4.1+ recommended over the legacy 'class'
}
v3 legacy form (still valid but superseded) :
module.exports = {
darkMode: 'class',
}
v4 :
@import"tailwindcss";
@custom-variant dark (&:where(.dark, .dark *));
Markup (both versions) :
<htmlclass="dark"><body><divclass="bg-white text-slate-900 dark:bg-slate-900 dark:text-white">
Reacts to the `.dark` class on <html>.
</div></body></html>
ALWAYS toggle the class on <html> (not <body>) for canonical Tailwind behavior. The :where(.dark, .dark *) selector matches BOTH the ancestor AND descendants, so the variant applies regardless of nesting depth.
@import"tailwindcss";
@custom-variant dark (&:where([data-theme=dark], [data-theme=dark] *));
Markup :
<htmldata-theme="dark"><body><divclass="bg-background text-foreground dark:bg-slate-900 dark:text-white">
Reacts to data-theme.
</div></body></html>
ALWAYS use this strategy when integrating with next-themes (attribute="data-theme" config), shadcn ui (which uses class by default but supports attribute via next-themes), or any system that already toggles a data attribute.
Pattern 4 : Theme-token swap (CSS variables)
ALWAYS pair this pattern with strategy 2 or 3 when a multi-theme design system exists. NEVER hand-write dark: variants on every utility in such a system : the variable swap covers the whole tree.
ALWAYS keep the same token names across light and dark; only the VALUES change inside the .dark selector. NEVER introduce a dark-primary token; that defeats the swap model.
Pattern 5 : The canonical JavaScript toggle (verbatim from v4 docs)
ALWAYS run this snippet AS EARLY AS POSSIBLE in the page lifecycle. Inline in <head> is the only safe location for SSR / static HTML; framework-specific equivalents below.
ALWAYS check localStorage.theme FIRST (explicit user choice), THEN fall back to prefers-color-scheme (OS preference). NEVER check the OS preference first; that ignores the user's explicit override.
Pattern 6 : Framework-specific toggle placement
Framework
Where the inline script goes
Next.js (App Router)
In app/layout.tsx<head> via a literal <script> element OR use next-themes package
Next.js (Pages Router)
In pages/_document.tsx<Head>
Vite + React
In index.html<head> before the <div id="root">
Astro
In the layout component <head>, before any client component
SvelteKit
In src/app.html<head>
Remix / React Router v7
In root.tsx<head>
Plain HTML
In <head> before the stylesheet link
ALWAYS place the inline script BEFORE any stylesheet link to guarantee the class is set when the first CSS parse happens. NEVER place it after the stylesheet : the browser may paint once with the wrong theme.
Anti-patterns (summary; full list in references/anti-patterns.md)
NEVER do these :
NEVER leave a v3 darkMode: 'class' config in a v4 project. v4 ignores the field silently; ALWAYS migrate to @custom-variant dark (...) in CSS.
NEVER run the toggle inside a useEffect / onMount / mounted() lifecycle hook : the wrong theme will flash on first paint.
NEVER toggle the class on <body> instead of <html> : the :where(.dark, .dark *) selector targets the ancestor chain; body.dark works only if every styled element is a descendant, which fails for portals.
NEVER assume dark: variants need configuration before they work in v4 : the default media behaviour is automatic; configuration is needed ONLY to switch from media to class or attribute strategy.
NEVER check prefers-color-scheme before localStorage in the toggle script : the user's explicit override must win over the OS preference.
NEVER write a darkMode: ['variant', ...] v3 config and a @custom-variant dark v4 directive in the same project. Pick one entrypoint per version.
