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CSS

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HTML 

              
                <input id="dark-mode" class="dark-mode-checkbox visually-hidden" type="checkbox">

<div class="theme-container grow">
  <label class="dark-mode-label" for="dark-mode">
    Dark mode
  </label>
  
  <div class="wrapper">
    <h1>CSS-only dark mode 🌚</h1>
    
    <p>This pen was written for the article <a href="https://kleinfreund.de/css-only-dark-mode/">CSS-only dark mode</a>.</p>

    <p>Using the <code>:checked</code> pseudo-class selector, one can style descendants and subsequent siblings of a checkbox when it is in checked state.</p>
    
    <p>Having both a checkbox for enabling dark mode and an element containing all other content on the page on the same level of the DOM tree allows us to offer a CSS-only dark mode of a web page.</p>
    
    <p>A very nice implementation of this technique can be found on <a href="https://muan.co/">Mu-An Chiou’s website</a>. Her’s is the foundation for this implementation, so all credit goes to her!  🎉</p>
    
    <p>I made some small changes additions:</p>
    
    <ul>
      <li>The dark mode checkbox has a focus style.</li>
      <li>The content area grows to the viewport’s height so that the background color fills the viewport completely.</li>
      <li>Theming is done by setting CSS custom properties in a context-sensitive way.</li>
    </ul>
  </div>
</div>
!

CSS 

              
                :root {
  /* Light theme */
  --c-light-text: #333;
  --c-light-background: #fff;
  --c-light-focus: deepskyblue;
  --c-light-interactive: mediumvioletred;

  /* Dark theme */
  --c-dark-text: #fff;
  --c-dark-background: #333;
  --c-dark-focus: deeppink;
  --c-dark-interactive: palegreen;
}

.theme-container {
  /* Make the light theme the default */
  --c-text: var(--c-light-text);
  --c-background: var(--c-light-background);
  --c-focus: var(--c-light-focus);
  --c-interactive: var(--c-light-interactive);

  padding: 1.5rem;
  color: var(--c-text);
  background-color: var(--c-background);
}

.dark-mode-checkbox:checked ~ .theme-container {
  /* Override the default theme */
  --c-text: var(--c-dark-text);
  --c-background: var(--c-dark-background);
  --c-focus: var(--c-dark-focus);
  --c-interactive: var(--c-dark-interactive);
}

.dark-mode-label {
  margin-bottom: 1em;
}

:focus,
.dark-mode-checkbox:focus ~ .theme-container .dark-mode-label {
  outline: 2px solid var(--c-focus);
}


/*
* Replace original checkbox
*/

.dark-mode-label::before {
  content: "\2610";
}

.dark-mode-checkbox:checked ~ .theme-container .dark-mode-label::before {
  content: "\2611";
}

/*
visibility-hidden utility class

Source: https://github.com/h5bp/html5-boilerplate

Hide only visually, but have it available for screen readers:
https://snook.ca/archives/html_and_css/hiding-content-for-accessibility

1. For long content, line feeds are not interpreted as spaces
   and small width causes content to wrap 1 word per line:
   https://medium.com/@jessebeach/beware-smushed-off-screen-accessible-text-5952a4c2cbfe
*/
.visually-hidden {
  position: absolute;
  overflow: hidden;
  clip: rect(0 0 0 0);
  width: 1px;
  height: 1px;
  margin: -1px;
  padding: 0;
  border: 0;
  white-space: nowrap; /* 1. */
}



/*
* Grow content to viewport
*/

/*
1. Allows the body’s children to grow to
   100% of the viewport’s height.
*/
html,
body {
  height: 100%; /* 1. */
}

/*
1. Allows the content area to grow to the viewport height.
*/
body {
  display: flex; /* 1. */
  flex-direction: column; /* 1. */
}

/*
1. Grows the content area to take up all the remaining height
   inside the body element.
*/
.grow {
  flex-grow: 1; /* 1. */
}



/*
* Basic styles
*/
body {
  margin: 0;
  font-family: Cambria, Cochin, Georgia, Times, 'Times New Roman', serif;
  font-size: 1.25em;
  line-height: 1.5;
}

a {
  color: var(--c-interactive);
}

h1 {
  margin-top: 0;
}

code {
  font-family: 'Fira Mono', Consolas, 'Liberation Mono', Menlo, Courier, monospace;
}

.wrapper {
  max-width: 600px;
}
!

JS 

              
                document.addEventListener('DOMContentLoaded', function () {
  const checkbox = document.querySelector('.dark-mode-checkbox');

  checkbox.checked = localStorage.getItem('darkMode') === 'true';

  checkbox.addEventListener('change', function (event) {
    localStorage.setItem('darkMode', event.currentTarget.checked);
  });
});
!
999px

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