Tailwind CSS Academy · Lesson

Building a Dark Mode Toggle

Implement a JavaScript toggle that adds and removes the dark class on the html element and persists the preference in localStorage.

Building a Dark Mode Toggle is a free Tailwind CSS Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Tailwind CSS Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

What the Toggle Needs to Do

A dark mode toggle needs to accomplish three things: read the user's saved preference (or OS preference as a fallback), apply the correct class to the <html> element on page load, and update the class and save the preference when the user clicks the toggle button. Getting the initial load right is critical — applying the class before the page renders prevents a flash of incorrect theme.

Preventing Flash of Wrong Theme

The most common dark mode bug is a flash of light theme when a user in dark mode refreshes the page. This happens because JavaScript runs after the HTML has already been parsed and rendered. The fix is to run a tiny inline <script> in the <head> — before any CSS or body content — that reads localStorage and immediately adds the dark class if needed.

<!-- Place this FIRST inside <head> -->
<script>
  if (localStorage.theme === 'dark' ||
      (!('theme' in localStorage) && window.matchMedia('(prefers-color-scheme: dark)').matches)) {
    document.documentElement.classList.add('dark');
  }
</script>

HTML Structure for the Toggle

The toggle button itself can be as simple as a <button> with sun and moon icons. Show the sun icon in dark mode (to switch to light) and the moon icon in light mode (to switch to dark). Use Tailwind's hidden and block with dark: variants to swap which icon is visible based on the current mode.

<button id="theme-toggle" class="p-2 rounded-lg bg-gray-200 dark:bg-gray-700">
  <!-- Moon icon (visible in light mode) -->
  <svg id="icon-moon" class="w-5 h-5 text-gray-800 dark:hidden" fill="currentColor" viewBox="0 0 20 20">
    <path d="M17.293 13.293A8 8 0 016.707 2.707a8.001 8.001 0 1010.586 10.586z" />
  </svg>
  <!-- Sun icon (visible in dark mode) -->
  <svg id="icon-sun" class="w-5 h-5 text-yellow-400 hidden dark:block" fill="currentColor" viewBox="0 0 20 20">
    <path d="M10 2a1 1 0 100 2 1 1 0 000-2zm0 14a1 1 0 100 2 1 1 0 000-2z" />
  </svg>
</button>

Writing the Toggle JavaScript

The toggle function checks whether the dark class is currently present on <html>. If it is, the function removes it and saves 'light' to localStorage. If it is not, the function adds it and saves 'dark'. This pattern is clean, reversible, and works without any external libraries or frameworks.

const toggle = document.getElementById('theme-toggle');

toggle.addEventListener('click', () => {
  const html = document.documentElement;

  if (html.classList.contains('dark')) {
    html.classList.remove('dark');
    localStorage.setItem('theme', 'light');
  } else {
    html.classList.add('dark');
    localStorage.setItem('theme', 'dark');
  }
});

Respecting OS Preference as Default

If the user has never interacted with your toggle, there is nothing in localStorage. In that case, you should fall back to the OS prefers-color-scheme media query. The window.matchMedia API lets you check this at runtime. If the OS is in dark mode and the user has not overridden it, apply the dark class so the initial experience matches their system setting.

function getInitialTheme() {
  const saved = localStorage.getItem('theme');
  if (saved) return saved;

  // No saved preference — use OS setting
  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
}

if (getInitialTheme() === 'dark') {
  document.documentElement.classList.add('dark');
}

Full Toggle Implementation

Putting it all together: the inline head script handles initial load without flash, the button HTML uses dark: variants to swap icons, and the click handler toggles the class and saves to localStorage. This complete pattern is the standard approach used by most Tailwind-based apps. It requires no dependencies and works in any environment.

<!-- In <head> (anti-flash) -->
<script>
  const saved = localStorage.getItem('theme');
  const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
  if (saved === 'dark' || (!saved && prefersDark)) {
    document.documentElement.classList.add('dark');
  }
</script>

<!-- In body JS -->
<script>
  document.getElementById('theme-toggle').addEventListener('click', () => {
    const isDark = document.documentElement.classList.toggle('dark');
    localStorage.setItem('theme', isDark ? 'dark' : 'light');
  });
</script>

Toggle Button With Smooth Transition

Add a satisfying animation to the theme switch by putting a transition-colors class on your body or root container. This makes all color changes animate smoothly instead of snapping instantly. Keep the transition duration short — around 200-300ms — to feel responsive. Apply it at the highest container level so all child elements inherit the smooth transition behavior.

<body class="bg-white dark:bg-gray-900 text-gray-900 dark:text-gray-100 transition-colors duration-200">
  <!-- All children now transition smoothly between light and dark -->
</body>

Toggle in a React Component

In React, manage the dark mode state with useState and synchronize it with the DOM and localStorage using useEffect. A common pattern is a custom useDarkMode hook that encapsulates all the logic. Trigger the DOM class change inside the effect so it stays in sync with the React state.

import { useState, useEffect } from 'react';

function useDarkMode() {
  const [dark, setDark] = useState(() => localStorage.getItem('theme') === 'dark');

  useEffect(() => {
    document.documentElement.classList.toggle('dark', dark);
    localStorage.setItem('theme', dark ? 'dark' : 'light');
  }, [dark]);

  return [dark, setDark];
}

Toggle in a Next.js App

In Next.js, the anti-flash script cannot be easily placed in <head> via a component because React hydration runs after the HTML is parsed. The recommended approach is to add the inline script via next/script with strategy='beforeInteractive', or to place it directly in a custom _document.js file inside the Head component. This ensures the script runs synchronously before the page renders.

// pages/_document.js (Next.js Pages Router)
import { Html, Head, Main, NextScript } from 'next/document';

export default function Document() {
  return (
    <Html>
      <Head>
        <script dangerouslySetInnerHTML={{ __html: `
          const t = localStorage.getItem('theme');
          const d = window.matchMedia('(prefers-color-scheme: dark)').matches;
          if (t === 'dark' || (!t && d)) document.documentElement.classList.add('dark');
        ` }} />
      </Head>
      <body><Main /><NextScript /></body>
    </Html>
  );
}

Listening to OS Changes at Runtime

Users sometimes change their OS theme while your app is open. You can listen for these changes using window.matchMedia's change event. Only apply the OS preference change if the user has not set an explicit preference in your app — otherwise you would override their in-app choice. This provides the most respectful user experience.

const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');

mediaQuery.addEventListener('change', (e) => {
  // Only auto-update if the user hasn't set an explicit preference
  if (!localStorage.getItem('theme')) {
    document.documentElement.classList.toggle('dark', e.matches);
  }
});

Toggle Accessibility Considerations

The dark mode toggle button should be keyboard accessible and have a meaningful aria-label that updates to reflect the current action. Use aria-pressed to communicate the current state to screen readers. Since only a visual icon changes, screen reader users need the label to understand what pressing the button will do.

<button
  id="theme-toggle"
  aria-label="Switch to dark mode"
  aria-pressed="false"
  class="p-2 rounded-lg bg-gray-200 dark:bg-gray-700"
>
  <!-- icons -->
</button>

<script>
  const btn = document.getElementById('theme-toggle');
  btn.addEventListener('click', () => {
    const isDark = document.documentElement.classList.toggle('dark');
    btn.setAttribute('aria-pressed', isDark);
    btn.setAttribute('aria-label', isDark ? 'Switch to light mode' : 'Switch to dark mode');
    localStorage.setItem('theme', isDark ? 'dark' : 'light');
  });
</script>

Quick Check

Test your understanding of building a dark mode toggle with Tailwind CSS.

Lesson Recap

In this lesson you learned: an inline head script prevents the flash of wrong theme on page load, classList.toggle('dark') combined with localStorage creates a persistent user toggle, and matchMedia lets you respect the OS preference as a default when no user preference is saved. Next up we apply dark mode to complex components like cards, navbars, and modals.

Frequently asked questions

Is the “Building a Dark Mode Toggle” lesson free?

Yes — the full text of “Building a Dark Mode Toggle” is free to read here on the web, and the Tailwind CSS Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Tailwind CSS Academy course, upgrade to CoddyKit PRO.

What will I learn in “Building a Dark Mode Toggle”?

Implement a JavaScript toggle that adds and removes the dark class on the html element and persists the preference in localStorage. You practise Tailwind CSS Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Tailwind CSS Academy?

No prior experience is required. Tailwind CSS Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Building a Dark Mode Toggle” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Tailwind CSS Academy lesson?

Yes. Every Tailwind CSS Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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