<html>

<head>
  <title>Light / Dark Theme Switcher in Vanilla JavaScript and CSS</title>
</head>

<body data-theme="">
  <header>
    <div class="logo">Brand</div>
    <nav>
      <ul>
        <li><a href="#home">Home</a></li>
        <li><a href="#news">News</a></li>
        <li><a href="#contact">Contact</a></li>
        <li><a href="#about">About</a></li>
      </ul>
    </nav>
    <button id="theme" onclick="switchTheme()">Dark</button>
  </header>
  <main>
    <h1>Chocolate Chip Muffins</h1>
    <p>
      A muffin is an individual-sized, baked product. It can refer to two distinct items, a part-raised flatbread that is baked and then cooked on a griddle (typically unsweetened) and a cupcake-like quickbread (often sweetened) that is chemically leavened and then baked in a mold. While quickbread muffins are often sweetened, there are savory varieties made with ingredients such as corn and cheese. The flatbread is of British or European derivation, and dates from at least the early 18th century, while the quickbread originated in North America during the 19th century. Both are common worldwide today.
    </p>
    <h2>Muffin top</h2>
    <p>The muffin top is the crisp upper part of the muffin, which has developed a "browned crust that's slightly singed around the edges".In 2018, McDonald's restaurant announced they were planning to sell muffin tops as part of its McCafe breakfast menu.
    </p>
  </main>
</body>

</html>
* {
  margin: 0;
  padding: 0;
}

:root {
  --font: Sans-Serif;
  --font-size-1: 16px;
  --font-size-2: 32px;
  --font-size-3: 48px;
  --font-size-4: 56px;
  --font-weight-1: 400;
  --font-weight-2: 700;
  --font-weight-3: 900;
  --white: white;
  --black: black;
}

[data-theme="dark"],
[data-theme="dark"] a {
  color: var(--white);
  background: var(--black);
}

[data-theme="light"] #theme {
  color: var(--white);
  background: var(--black);
}

body {
  font-family: var(--font);
  font-size: var(--font-size-1);
  max-width: 960px;
  margin: 0 auto;
}

header {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

header .logo {
  font-size: var(--font-size-2);
  font-weight: var(--font-weight-2);
}

nav ul {
  list-style-type: none;
  display: flex;
}

nav ul li {
  margin-right: 10px;
}

header #theme {
  border: 0;
  padding: 5px 10px;
}

h1,
h2 {
  margin-top: 30px;
  margin-bottom: 10px;
}

h1 {
  font-size: var(--font-size-4);
}

h2 {
  font-size: var(--font-size-3);
}
const themeSwitcher = document.querySelector('#theme');

function switchTheme() {
  // toggle theme
  const toggleValue = localStorage.getItem('theme') === 'dark' ? 'light' : 'dark';
  
  // update localstorage value;
  localStorage.setItem('theme', toggleValue);
  
  // update theme
  setTheme();
}

function setTheme() {
  // set current theme - default to `light` on first load and set `theme` property in localStorage.
  const currentTheme = localStorage.getItem('theme') || 'light';
  
  // update current theme value on `data-theme` attribute
  document.body.dataset.theme = currentTheme;

  // update inner text of button dynamically based on current theme
  themeSwitcher.innerText = currentTheme === 'light' ? 'Dark' : 'Light';
}

// set theme on inital load
setTheme();

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.