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HTML

About HTML Preprocessors

HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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Adding Classes

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

About the <head>

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. If you need things in the <head> of the document, put that code here.

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Insecure Resource

The resource you are linking to is using the 'http' protocol, which may not work when the browser is using https.

CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

We offer two popular choices: Autoprefixer (which processes your CSS server-side) and -prefix-free (which applies prefixes via a script, client-side).

Add External Stylesheets/Pens

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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Packages

Add Packages

Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.

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Using packages here is powered by esm.sh, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ESM usage.

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HTML 

              
                <h1>Custom color mode toggler for Bootstrap v5.3.0+</h1>

<ul class="switches">
  <li>
    <input type="radio" id="light" name="theme-mode" checked>
    <label for="light">
      <span>Light</span>
      <span></span>
    </label>
  </li>
  <li>
    <input type="radio" id="dark" name="theme-mode">
    <label for="dark">
      <span>Dark</span>
      <span></span>
    </label>
  </li>
  <li>
    <input type="radio" id="auto" name="theme-mode">
    <label for="auto">
      <span>Auto</span>
      <span></span>
    </label>
  </li>
</ul>

<footer class="page-footer">
  <span>made by </span>
  <a href="https://georgemartsoukos.com/" target="_blank">
    <img width="24" height="24" src="https://assets.codepen.io/162656/george-martsoukos-small-logo.svg" alt="George Martsoukos logo">
  </a>
</footer>
!

CSS 

              
                /* RESET STYLES & VARIABLES
–––––––––––––––––––––––––––––––––––––––––––––––––– */
:root {
  --white-pearl: #ede1d2;
  --gray: #e8e8ec;
  --slate-gray: #789392;
}

ul {
  list-style: none;
}

h1 {
  font-size: 26px;
}

body {
  margin: 30px auto;
  text-align: center;
}

/* MAIN STYLES
–––––––––––––––––––––––––––––––––––––––––––––––––– */
.switches {
  display: inline-block;
  padding: 0;
  border: 1px solid var(--gray);
  margin: 10px 0 0;
  border-radius: 6px;
}

.switches li {
  position: relative;
}

.switches li:not(:last-child) {
  border-bottom: 1px solid var(--gray);
}

.switches li [type="radio"] {
  position: absolute;
  left: -9999px;
}

.switches label {
  display: grid;
  grid-template-columns: 40px auto;
  align-items: center;
  gap: 10px;
  padding: 20px;
  cursor: pointer;
}

.switches span {
  flex-shrink: 0;
}

.switches span:empty {
  position: relative;
  width: 50px;
  height: 26px;
  border-radius: 15px;
  background: var(--gray);
  transition: all 0.3s;
}

.switches span:empty::before,
.switches span:empty::after {
  content: "";
  position: absolute;
}

.switches span:empty::before {
  top: 1px;
  left: 1px;
  width: 24px;
  height: 24px;
  background: var(--slate-gray);
  border-radius: 50%;
  z-index: 1;
  transition: transform 0.3s;
}

.switches span:empty::after {
  top: 50%;
  transform: translateY(-50%);
  width: 14px;
  height: 14px;
  left: 6px;
  background-size: 14px 14px;
}

.switches [type="radio"]:checked + label span:empty {
  background: var(--white-pearl);
}

.switches [type="radio"]:checked + label span:empty::before {
  transform: translateX(24px);
}

.switches li:nth-child(1) [type="radio"]:checked + label span:empty::after {
  background-image: url("data:image/svg+xml;base64,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");
}

.switches li:nth-child(2) [type="radio"]:checked + label span:empty::after {
  background-image: url("data:image/svg+xml;base64,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");
}

.switches li:nth-child(3) [type="radio"]:checked + label span:empty::after {
  background-image: url("data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHZpZXdCb3g9IjAgMCA1MTIgNTEyIj48IS0tIUZvbnQgQXdlc29tZSBGcmVlIDYuNy4yIGJ5IEBmb250YXdlc29tZSAtIGh0dHBzOi8vZm9udGF3ZXNvbWUuY29tIExpY2Vuc2UgLSBodHRwczovL2ZvbnRhd2Vzb21lLmNvbS9saWNlbnNlL2ZyZWUgQ29weXJpZ2h0IDIwMjQgRm9udGljb25zLCBJbmMuLS0+PHBhdGggZD0iTTQ0OCAyNTZjMC0xMDYtODYtMTkyLTE5Mi0xOTJsMCAzODRjMTA2IDAgMTkyLTg2IDE5Mi0xOTJ6TTAgMjU2YTI1NiAyNTYgMCAxIDEgNTEyIDBBMjU2IDI1NiAwIDEgMSAwIDI1NnoiLz48L3N2Zz4=");
}

/* FOOTER STYLES
–––––––––––––––––––––––––––––––––––––––––––––––––– */
.page-footer {
  position: fixed;
  right: 0;
  bottom: 50px;
  display: flex;
  align-items: center;
  padding: 5px;
  font-size: 0.9em;
  color: var(--text-color);
  background: var(--bg-color);
}

.page-footer a {
  display: flex;
  margin-left: 4px;
}
!

JS 

              
                const html = document.documentElement;
const switches = document.querySelector(".switches");
const inputs = switches.querySelectorAll("input");

if (localStorage.getItem("bs-dark-mode")) {
  html.setAttribute("data-bs-theme", "dark");
}

if (localStorage.getItem("bs-selected-radio")) {
  switches.querySelector(
    `#${localStorage.getItem("bs-selected-radio")}`
  ).checked = "true";
}

const setTheme = (theme) => {
  if (theme === "dark") {
    html.setAttribute("data-bs-theme", "dark");
    localStorage.setItem("bs-dark-mode", "true");
  } else {
    html.removeAttribute("data-bs-theme");
    localStorage.removeItem("bs-dark-mode");
  }
};

const handleMediaChange = (e) => {
  if (switches.querySelector('[type="radio"]:checked').id === "auto") {
    setTheme(e.matches ? "dark" : "light");
  }
};

const handleInputChange = (e) => {
  const themeMode = e.target.id;
  if (
    themeMode === "dark" ||
    (themeMode === "auto" &&
      window.matchMedia("(prefers-color-scheme: dark)").matches)
  ) {
    setTheme("dark");
  } else {
    setTheme("light");
  }
  localStorage.setItem("bs-selected-radio", themeMode);
};

window
  .matchMedia("(prefers-color-scheme: dark)")
  .addEventListener("change", handleMediaChange);

inputs.forEach((input) => input.addEventListener("input", handleInputChange));
!
999px

Console