<!DOCTYPE html>
<html lang="RU" class="theme-light">
  
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Переключатель темных и светлых тем
</title>
  <link rel="stylesheet" href="./styles.css" type="text/css" />
</head>

<body>
  <div class="container">
    <h1>Переключатель темных и светлых тем
</h1>
    <label id="switch" class="switch">
            <input type="checkbox" onchange="toggleTheme()" id="slider">
            <span class="slider round"></span>
        </label>
    <!-- <button id="switch" onclick="toggleTheme()">Switch</button> -->
  </div>
  <script>
  </script>
</body>

</html>
html,
body {
  margin: 0;
  padding: 0;
  height: 100%;
  width: 100%;
}
.theme-light {
  --color-primary: #0060df;
  --color-secondary: #fbfbfe;
  --color-accent: #fd6f53;
  --font-color: #000000;
}
.theme-dark {
  --color-primary: #17ed90;
  --color-secondary: #2a2c2d;
  --color-accent: #12cdea;
  --font-color: #ffffff;
}
.container {
  display: flex;
  width: 100%;
  height: 100%;
  background: var(--color-secondary);
  flex-direction: column;
  justify-content: center;
  align-items: center;
}
.container h1 {
  color: var(--font-color);
  font-family: sans-serif;
}
.container button {
  color: var(--font-color);
  background: var(--color-primary);
  padding: 10px 20px;
  border: 0;
  border-radius: 5px;
}

/* The switch - the box around the slider */
.switch {
  position: relative;
  display: inline-block;
  width: 60px;
  height: 34px;
}

/* Hide default HTML checkbox */
.switch input {
  opacity: 0;
  width: 0;
  height: 0;
}

/* The slider */
.slider {
  position: absolute;
  cursor: pointer;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: #ccc;
  -webkit-transition: 0.4s;
  transition: 0.4s;
}

.slider:before {
  position: absolute;
  content: "";
  height: 40px;
  width: 40px;
  left: 0px;
  bottom: 4px;
  top: 0;
  bottom: 0;
  margin: auto 0;
  -webkit-transition: 0.4s;
  transition: 0.4s;
  box-shadow: 0 0px 15px #2020203d;
  background: white url('https://i.ibb.co/FxzBYR9/night.png');
  background-repeat: no-repeat;
  background-position: center;
}

input:checked + .slider {
  background-color: #2196f3;
}

input:focus + .slider {
  box-shadow: 0 0 1px #2196f3;
}

input:checked + .slider:before {
  -webkit-transform: translateX(24px);
  -ms-transform: translateX(24px);
  transform: translateX(24px);
  background: white url('https://i.ibb.co/7JfqXxB/sunny.png');
  background-repeat: no-repeat;
  background-position: center;
}

/* Rounded sliders */
.slider.round {
  border-radius: 34px;
}

.slider.round:before {
  border-radius: 50%;
}
 // function to set a given theme/color-scheme
        function setTheme(themeName) {
            localStorage.setItem('theme', themeName);
            document.documentElement.className = themeName;
        }

        // function to toggle between light and dark theme
        function toggleTheme() {
            if (localStorage.getItem('theme') === 'theme-dark') {
                setTheme('theme-light');
            } else {
                setTheme('theme-dark');
            }
        }

        // Immediately invoked function to set the theme on initial load
        (function () {
            if (localStorage.getItem('theme') === 'theme-dark') {
                setTheme('theme-dark');
                document.getElementById('slider').checked = false;
            } else {
                setTheme('theme-light');
              document.getElementById('slider').checked = true;
            }
        })();

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.