Class-based dark mode strategy in Tailwind

Introduction

Dark mode helps reduce eye strain in low light. Using a class to switch modes lets you control when dark mode is on.

You want users to toggle between light and dark mode manually.

You want to apply dark mode only to parts of your website.

You want to avoid relying on the user's system settings for dark mode.

You want to test dark mode easily during development.

You want consistent dark mode styling across your site.

Syntax

Tailwind

<!-- Add 'dark' class to a parent element -->
<div class="dark">
  <p class="bg-white dark:bg-gray-800 text-black dark:text-white">Hello, dark mode!</p>
</div>

The dark class on a parent triggers dark styles inside it.

Use dark: prefix in Tailwind to style elements for dark mode.

Examples

The button background and text color change when the dark class is present on the parent.

Tailwind

<div class="dark">
  <button class="bg-gray-200 dark:bg-gray-700 text-black dark:text-white">Click me</button>
</div>

Applying dark class on body switches the whole page to dark mode styles.

Tailwind

<body class="dark">
  <h1 class="text-gray-900 dark:text-gray-100">Welcome</h1>
</body>

If no dark class is present, dark styles won't apply.

Tailwind

<div>
  <p class="bg-white dark:bg-gray-800">Light background by default, dark background if parent has <code>dark</code> class.</p>
</div>

Sample Program

This example uses Tailwind's class-based dark mode. Clicking the button toggles the dark class on the html element. The box background and text colors change accordingly.

Tailwind

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1" />
  <title>Class-based Dark Mode Example</title>
  <script src="https://cdn.tailwindcss.com"></script>
  <script>
    tailwind.config = {
      darkMode: 'class'
    }
  </script>
</head>
<body class="p-6">
  <button id="toggleDark" class="mb-4 px-4 py-2 bg-gray-300 dark:bg-gray-700 text-black dark:text-white rounded">
    Toggle Dark Mode
  </button>
  <div id="container" class="p-6 bg-white dark:bg-gray-900 text-black dark:text-white rounded">
    <h1 class="text-2xl font-bold mb-2">Hello, World!</h1>
    <p>This box changes colors when dark mode is toggled.</p>
  </div>
  <script>
    const button = document.getElementById('toggleDark');
    button.addEventListener('click', () => {
      document.documentElement.classList.toggle('dark');
    });
  </script>
</body>
</html>

Important Notes

Make sure to set darkMode: 'class' in your Tailwind config to use this strategy.

Toggle the dark class on a parent element like html or body to switch modes.

Use semantic HTML and ensure good color contrast for accessibility in both modes.

Summary

Class-based dark mode uses a dark class on a parent to control dark styles.

Tailwind's dark: prefix applies styles only when the dark class is present.

This method lets users toggle dark mode manually and gives you full control over styling.

Practice

1. What does the dark: prefix in Tailwind CSS do when using class-based dark mode?

easy

A. Applies styles only when a parent element has the dark class.

B. Automatically detects system dark mode and applies styles.

C. Adds a dark background color to all elements.

D. Disables all light mode styles on the page.

Solution

1. Step 1: Understand the dark: prefix role

   The dark: prefix in Tailwind applies styles only when the dark class is present on a parent element.

2. Step 2: Compare with other options

   It does not auto-detect system settings or disable light styles; it depends on the dark class presence.

3. Final Answer:

   Applies styles only when a parent element has the dark class. -> Option A

4. Quick Check:

   dark: prefix = styles with dark class [OK]

Hint: Remember: dark: works only if dark class exists [OK]

Common Mistakes:

• Thinking dark: auto-detects system dark mode
• Assuming it changes styles without the dark class
• Confusing dark: with global dark mode toggle

2. Which of the following is the correct way to enable dark mode using class strategy in Tailwind CSS?

easy

A. "darkMode: 'media'" in tailwind.config.js

B. "darkMode: 'class'" in tailwind.config.js

C. Adding dark attribute to body tag

D. Using dark-mode class instead of dark

Solution

1. Step 1: Check Tailwind config for dark mode strategy

   To use class-based dark mode, you must set darkMode: 'class' in tailwind.config.js.

2. Step 2: Eliminate other options

   darkMode: 'media' uses system preference, not class. The dark class must be added manually; no dark attribute exists. The class name must be exactly dark.

3. Final Answer:

   "darkMode: 'class'" in tailwind.config.js -> Option B

4. Quick Check:

   Config setting for class strategy = 'class' [OK]

Hint: Set darkMode to 'class' in config for class-based dark mode [OK]

Common Mistakes:

• Using 'media' instead of 'class' in config
• Adding wrong attribute instead of class
• Using incorrect class name like 'dark-mode'

3. Given this HTML snippet:

<html class="dark">
  <body>
    <div class="bg-white dark:bg-gray-800 text-black dark:text-white p-4">Hello</div>
  </body>
</html>

What background and text colors will the <div> have when rendered?

medium

A. Background white, text black

B. Background gray-800, text black

C. Background gray-800, text white

D. Background white, text white

Solution

1. Step 1: Identify presence of dark class

   The <html> tag has the dark class, so all dark: styles apply.

2. Step 2: Apply styles based on class

   The div has bg-white and text-black normally, but with dark:bg-gray-800 and dark:text-white when dark class is present. So background is gray-800 and text is white.

3. Final Answer:

   Background gray-800, text white -> Option C

4. Quick Check:

   Dark class present = dark styles apply [OK]

Hint: If dark class exists, dark: styles override [OK]

Common Mistakes:

• Ignoring the dark class on html
• Mixing normal and dark styles incorrectly
• Assuming no style changes without JavaScript

4. You want to toggle dark mode by adding/removing the dark class on body. Which of these JavaScript snippets correctly toggles dark mode?

medium

A. document.body.setAttribute('dark', true)

B. document.body.toggleClass('dark')

C. document.body.className = 'dark'

D. document.body.classList.toggle('dark')

Solution

1. Step 1: Understand class toggling in JavaScript

   The correct method to toggle a class is classList.toggle('className').

2. Step 2: Check each option

   document.body.classList.toggle('dark') uses classList.toggle('dark'), which is correct. document.body.toggleClass('dark') uses a non-existent toggleClass method. document.body.className = 'dark' overwrites all classes, removing others. document.body.setAttribute('dark', true) sets an attribute, not a class.

3. Final Answer:

   document.body.classList.toggle('dark') -> Option D

4. Quick Check:

   Toggle class method = classList.toggle [OK]

Hint: Use classList.toggle('dark') to switch dark mode on/off [OK]

Common Mistakes:

• Using jQuery method toggleClass in plain JS
• Replacing className instead of toggling
• Setting attribute instead of class

5. You want to create a button that toggles dark mode by adding/removing the dark class on the html element. Which React code snippet correctly implements this using Tailwind's class-based dark mode?

hard

A. const toggleDark = () => { document.documentElement.classList.toggle('dark'); };

return Toggle Dark;

B. const toggleDark = () => { document.body.classList.add('dark'); };

return Toggle Dark;

C. const toggleDark = () => { document.documentElement.setAttribute('dark', 'true'); };

return Toggle Dark;

D. const toggleDark = () => { document.body.className = 'dark'; };

return Toggle Dark;

Solution

1. Step 1: Identify correct element and method for toggling

   Tailwind's class-based dark mode expects the dark class on the html element (document.documentElement). Toggling means adding or removing the class, so classList.toggle is best.

2. Step 2: Evaluate each option

   const toggleDark = () => { document.documentElement.classList.toggle('dark'); }; return Toggle Dark; toggles dark on html correctly. const toggleDark = () => { document.body.classList.add('dark'); }; return Toggle Dark; only adds dark to body (no toggle). const toggleDark = () => { document.documentElement.setAttribute('dark', 'true'); }; return Toggle Dark; sets an attribute instead of class. const toggleDark = () => { document.body.className = 'dark'; }; return Toggle Dark; overwrites all classes on body, removing others.

3. Final Answer:

   const toggleDark = () => { document.documentElement.classList.toggle('dark'); }; return <button onClick={toggleDark}>Toggle Dark</button>; -> Option A

4. Quick Check:

   Toggle dark class on html with classList.toggle [OK]

Hint: Toggle dark class on html using classList.toggle [OK]

Common Mistakes:

• Adding class without toggling
• Using wrong element like body
• Setting attribute instead of class
• Overwriting all classes instead of toggling