Class-based dark mode strategy in Tailwind - Mini Project: Build & Apply

Class-based Dark Mode Strategy with Tailwind CSS

📖 Scenario: You are building a simple webpage that supports dark mode using Tailwind CSS. Instead of relying on the user's system preference, you want to control dark mode by toggling a CSS class on the root element.

🎯 Goal: Create a webpage with a button that toggles dark mode by adding or removing the dark class on the <html> element. The page background and text colors should change accordingly using Tailwind's class-based dark mode strategy.

📋 What You'll Learn

Use Tailwind CSS with class strategy for dark mode

Create a button that toggles the dark class on the <html> element

Use Tailwind classes to style background and text colors differently in light and dark modes

Ensure the page is accessible with proper button labeling

Make the page responsive and visually clear in both modes

💡 Why This Matters

🌍 Real World

Many modern websites let users switch between light and dark themes manually. This project shows how to implement that with Tailwind CSS using a class-based approach.

💼 Career

Understanding dark mode toggling and Tailwind CSS configuration is useful for frontend developers building user-friendly, accessible, and modern web interfaces.

1

Create the basic HTML structure with Tailwind CSS

Create an HTML file with lang="en", charset="UTF-8", and a viewport meta tag. Inside the <body>, add a <main> element with a heading <h1> that says "Welcome to Dark Mode Demo". Use Tailwind classes bg-white and text-black on <body> for light mode styling.

Tailwind

  
  
  Dark Mode Demo
  


  
    Welcome to Dark Mode Demo
  

Hint

Remember to include the lang attribute on <html> and use Tailwind classes bg-white and text-black on <body>.

2

Add a configuration for dark mode class strategy

Add a <script> tag inside the <head> that configures Tailwind CSS to use the class strategy for dark mode by setting tailwind.config = { darkMode: 'class' }.

Tailwind

  
  
  Dark Mode Demo
  
    // Configure Tailwind dark mode to 'class'
  
  


  
    Welcome to Dark Mode Demo
  

Hint

Place the Tailwind config script before loading Tailwind CSS script.

3

Add a button to toggle dark mode class on <html>

Inside the <main> element, add a <button> with id="toggle-dark-mode" and accessible label Toggle dark mode. Below the button, add a <script> that adds a click event listener to this button. The listener should toggle the dark class on the document.documentElement (the <html> element).

Tailwind

  
  
  Dark Mode Demo
  
    tailwind.config = { darkMode: 'class' };
  
  


  
    Welcome to Dark Mode Demo
    
  
  
    // Add event listener to toggle dark mode class
  

Hint

Use document.getElementById to get the button and add a click event listener that toggles the dark class on document.documentElement.

4

Apply Tailwind dark mode classes for background and text colors

Update the <body> element classes to use bg-white and text-black for light mode, and add dark:bg-gray-900 and dark:text-white for dark mode styling. Also, add Tailwind padding p-6 to <main> for spacing.

Tailwind

  
  
  Dark Mode Demo
  
    tailwind.config = { darkMode: 'class' };
  
  


  
    Welcome to Dark Mode Demo
    Toggle Dark Mode
  
  
    const button = document.getElementById('toggle-dark-mode');
    button.addEventListener('click', () => {
      document.documentElement.classList.toggle('dark');
    });
  

Hint

Add the dark mode classes dark:bg-gray-900 and dark:text-white to the <body> element and add padding p-6 to <main>.

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