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HTML 

              
                <div class="container">
  <section class="header">
    <h1>Light / Dark Mode Toggle</h1>
    <label class="switch">
      <input type="checkbox" id="theme-toggle">
      <span class="slider"></span>
    </label>
  </section>
  <p>This example demonstrates a toggle switch that adapts to your system’s dark or light mode preference using CSS variables.</p>

  <ul>
    <li><strong>System Preference :</strong> The toggle and page theme automatically align with your system’s mode, providing an intuitive default.</li>
    <li><strong>Switch behavior:</strong> Click the switch to change the page’s background and text colors between light and dark mode.</li>
  </ul>
  <p>Try toggling the switch and adjusting your system’s dark mode setting to see the effect in action!</p>
  <a href="https://www.thingsaboutweb.dev/en/posts/css-variables-part-2">See full article here</a>
</div>
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CSS 

              
                /* Define CSS variables for color modes */
:root {
  --ON: initial; /* Default state variable to use for switching colors */
  --OFF: ; /* Alternative state variable for switching colors */
  
  /* Set default color variables based on light mode */
  --light: var(--ON);
  --dark: var(--OFF);
  
  /* Define custom properties for colors used in light and dark modes */
  --background-color: var(--light, #fbfbfb) var(--dark, #121212);
  --container-background-color: var(--light, #ebebeb) var(--dark, #555555);
  --headline-color: var(--light, #0077ee) var(--dark, #94b2e6);
  --text-color: var(--light, #333333) var(--dark, #e0e0e0);
}

/* Switch color modes when the checkbox (toggle) is checked */
:root:has(input[type="checkbox"]:checked) {
  --light: var(--OFF);
  --dark: var(--ON);
}

/* Invert the color behavior for system dark mode */
@media (prefers-color-scheme: dark) {
  :root {
    --light: var(--OFF);
    --dark: var(--ON);
  }

  /* Toggle to light mode colors when checkbox is checked in dark mode */
  :root:has(input[type="checkbox"]:checked) {
    --light: var(--ON);
    --dark: var(--OFF);
  }
}

/* Body styles with background and text color transitions */
body {
  background-color: var(--background-color);
  color: var(--text-color);
  transition: background-color 0.5s, color 0.5s;
}

/* Container styling with background color based on mode */
.container {
  background-color: var(--container-background-color);
  color: var(--text-color);
  margin: 5em auto;
  padding: 2em;
  border-radius: 15px;
  width: 30em;
  max-width: 80vw;
  transition: background-color 0.5s;
}

/* Header styling for a flex layout between toggle switch and title */
.header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  max-width: 100%;
}

/* Heading color based on mode with transition effect */
h1 {
  color: var(--headline-color);
  transition: color 0.5s;
}

/* Toggle Switch Styles */
.switch {
  position: relative;
  display: inline-block;
  width: 60px;
  height: 34px;
}

/* Hide the checkbox element to style a custom switch */
.switch input {
  opacity: 0;
  width: 0;
  height: 0;
}

/* Slider styling for the switch background */
.slider {
  position: absolute;
  cursor: pointer;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: var(--slider-bg, #ccc);
  transition: 0.4s;
  border-radius: 34px;
}

/* Slider knob styling */
.slider:before {
  position: absolute;
  content: "";
  height: 26px;
  width: 26px;
  left: 4px;
  bottom: 4px;
  background-color: white;
  transition: 0.4s;
  border-radius: 50%;
}

/* Dark mode styles: make the switch look "checked" by default */
@media (prefers-color-scheme: dark) {
  .slider {
    background-color: #94b2e6;
  }
  .slider:before {
    transform: translateX(26px); /* Move knob to the right */
  }

  /* Invert checked state in dark mode to look "unchecked" */
  input:checked + .slider {
    background-color: #ccc;
  }
  input:checked + .slider:before {
    transform: translateX(0); /* Move knob to the left */
  }
}

/* Light mode styles: make the switch look "unchecked" by default */
@media (prefers-color-scheme: light) {
  .slider {
    background-color: #ccc;
  }
  .slider:before {
    transform: translateX(0); /* Knob on the left */
  }

  /* Invert checked state in light mode to look "checked" */
  input:checked + .slider {
    background-color: #94b2e6;
  }
  input:checked + .slider:before {
    transform: translateX(26px); /* Move knob to the right */
  }
}
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