Build a CSS Dark Mode Toggle Switch (Source Code)

Dark mode has become an expected feature on modern websites. In this tutorial, you’ll implement a polished animated toggle switch that remembers the user’s preference across page loads using localStorage.

CSS Custom Properties (Variables)

The key to easy dark mode is using CSS variables on the :root that get overridden on [data-theme="dark"]:

:root {
  --bg:        #ffffff;
  --surface:   #f9fafb;
  --text:      #111827;
  --text-muted:#6b7280;
  --border:    #e5e7eb;
  --primary:   #04AA6D;
  --shadow:    rgba(0, 0, 0, 0.08);
}

[data-theme="dark"] {
  --bg:        #0f172a;
  --surface:   #1e293b;
  --text:      #f1f5f9;
  --text-muted:#94a3b8;
  --border:    rgba(255, 255, 255, 0.08);
  --primary:   #34d399;
  --shadow:    rgba(0, 0, 0, 0.4);
}

/* All elements use variables instead of hardcoded colors */
body {
  background: var(--bg);
  color: var(--text);
  transition: background 0.3s ease, color 0.3s ease;
}

The Toggle Switch HTML

<label class="toggle-switch" for="themeToggle" aria-label="Toggle dark mode">
  <input type="checkbox" id="themeToggle" />
  <div class="toggle-track">
    <span class="toggle-icon light">☀️</span>
    <span class="toggle-icon dark">🌙</span>
    <div class="toggle-thumb"></div>
  </div>
</label>

Toggle Switch CSS

.toggle-switch { cursor: pointer; display: inline-flex; align-items: center; }
.toggle-switch input { display: none; }

.toggle-track {
  width: 64px;
  height: 32px;
  background: #e5e7eb;
  border-radius: 32px;
  position: relative;
  transition: background .3s;
  display: flex;
  align-items: center;
  padding: 4px;
}
.toggle-switch input:checked + .toggle-track {
  background: #1e293b;
}

.toggle-thumb {
  width: 24px;
  height: 24px;
  border-radius: 50%;
  background: #fff;
  box-shadow: 0 2px 6px rgba(0,0,0,0.2);
  position: absolute;
  left: 4px;
  transition: transform .3s cubic-bezier(0.34, 1.56, 0.64, 1);
}
.toggle-switch input:checked + .toggle-track .toggle-thumb {
  transform: translateX(32px);
}

.toggle-icon {
  position: absolute;
  font-size: 14px;
  transition: opacity .2s;
}
.toggle-icon.light { left: 6px; }
.toggle-icon.dark  { right: 6px; }

/* Show correct icon based on state */
.toggle-icon.dark   { opacity: 0; }
input:checked + .toggle-track .toggle-icon.light { opacity: 0; }
input:checked + .toggle-track .toggle-icon.dark  { opacity: 1; }

JavaScript — Toggle + Persist

const toggle = document.getElementById('themeToggle');
const root   = document.documentElement;

// Apply saved preference on load
const saved = localStorage.getItem('theme') || 'light';
root.setAttribute('data-theme', saved);
toggle.checked = saved === 'dark';

// Listen for toggle change
toggle.addEventListener('change', () => {
  const theme = toggle.checked ? 'dark' : 'light';
  root.setAttribute('data-theme', theme);
  localStorage.setItem('theme', theme);
});

Respecting System Preference

// On first visit (no saved preference), use the OS setting
if (!localStorage.getItem('theme')) {
  const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
  const theme = prefersDark ? 'dark' : 'light';
  root.setAttribute('data-theme', theme);
  toggle.checked = prefersDark;
}

// Also react to live OS theme changes
window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', e => {
  if (!localStorage.getItem('theme')) {
    root.setAttribute('data-theme', e.matches ? 'dark' : 'light');
    toggle.checked = e.matches;
  }
});

Best Practice: Always check prefers-color-scheme as the default — it respects the user’s OS setting before they’ve ever interacted with your site.