<body x-data="{
    darkMode: true,
    toggleTheme() {
      this.darkMode = !this.darkMode;
    }
  }"
        :class="[darkMode ? 'ui-dark-theme' : 'ui-light-theme']">
  
    <header class="l-header">
      <h1><span>Темний режим</span></h1>
    </header>
  
    <div class="l-container">
      <div class="c-theme-switch" @click="toggleTheme()">
        <div class="c-theme-switch__toggle"></div>
      </div>
  
    </div>
  </body>
@import url('https://fonts.googleapis.com/css?family=Montserrat:100,100i,200,200i,300,300i,400,400i,500,500i,600,600i,700,700i,800,800i,900,900i');

.ui-dark-theme {
  --color-text: #fff;
  --color-custom: #ffeba7;
  --color-bg: #26242e;
  --color-primary: #ffeba7;
  --color-switch-theme: #fff;
}

.ui-light-theme {
  --color-text: #26242e;
  --color-custom: #ff3671;
  --color-bg: #fff;
  --color-primary: #ff3671;
  --color-switch-theme: #26242e;

  .c-theme-switch {
    .c-theme-switch__toggle {
      transform: translateX(0px);
    }
  }
}

/* ****************** */

body {
  height: 100vh;
  overflow: hidden;
  background: var(--color-bg);
  font-family: 'Oswald', sans-serif;
  transition: 500ms background ease;
}

body,
.l-container {
  display: flex;
  align-items: center;
  justify-content: center;
  flex-direction: column;
}

.l-header {
  margin-bottom: 12px;
  color: var(--color-text);
  
  h1 {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    text-transform: uppercase;
    font-family: Montserrat, sans-serif;
    font-weight: 900;
    font-size: 52px;
    color: var(--color-custom);
  }
}

 .c-theme-switch {
  position: relative;
  width: 70px;
  height: 30px;
  border-radius: 50px;
  color: var(--color-text);
  border: 1px solid var(--color-switch-theme);
  cursor: pointer;

  .c-theme-switch__toggle {
    width: 24px;
    height: 24px;
    background: var(--color-primary);
    border-radius: 100%;
    position: absolute;
    top: 3px;
    left: 4px;
    transform: translateX(37px);
    transition: 500ms all ease;
  }
}
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External CSS

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External JavaScript

  1. https://cdn.jsdelivr.net/gh/alpinejs/alpine@v2.3.5/dist/alpine.min.js