Implement Dark Mode in 5 Minutes with CSS Variables

Dark mode isn't a "feature" anymore; it's an accessibility requirement. And thanks to CSS Variables, it's trivial to implement.

Step 1: Define Variables

Don't set colors directly (color: black). Set semantic variables.

:root {
  /* Default (Light Mode) */
  --bg-primary: #ffffff;
  --text-primary: #1a1a1a;
  --accent: #2563eb;
  --card-bg: #f3f4f6;
}

Step 2: Media Query Magic

Use @media (prefers-color-scheme: dark) to override those variables.

@media (prefers-color-scheme: dark) {
  :root {
    --bg-primary: #111827;
    --text-primary: #f9fafb;
    --accent: #60a5fa;
    --card-bg: #1f2937;
  }
}

Step 3: Use the Variables

Now, write your CSS once.

body {
  background-color: var(--bg-primary);
  color: var(--text-primary);
  transition: background-color 0.3s ease, color 0.3s ease;
}

.card {
  background: var(--card-bg);
  border: 1px solid var(--accent);
}

Adding a Toggle Button (The User Override)

System preference is great, but users like control. To support a manual toggle, use a data attribute on the <html> tag.

/* Force Dark Mode when attribute is present */
[data-theme="dark"] {
  --bg-primary: #111827;
  --text-primary: #f9fafb;
  /* ... re-declare dark variables */
}

Then toggle it with JS:

document.documentElement.setAttribute('data-theme', 'dark');

Test Your Understanding

Quick Quiz

Which CSS media query detects if the user has requested the system use a light or dark color theme?

Conclusion

CSS Variables (--var) are reactive. When the media query changes (e.g., sunset triggers system dark mode), your entire site updates instantly without a page reload.