HTMLify

@import url("https://fonts.googleapis.com/css2?family=Montserrat&display=swap");

* {box-sizing: border-box;}

body {
  font-family: "Montserrat", sans-serif;
  background-color: #fff;
  display: flex;
  justify-content: center;
  align-items: center;
  flex-direction: column;
  text-align: center;
  min-height: 100vh;
  margin: 0;
  transition: background 0.2s linear;
}

body.dark {background-color: #292c35;} /* #9b59b6 */

body.dark h1, body.dark .support a {color: #fff;}

.checkbox {
  opacity: 0;
  position: absolute;
}

.checkbox-label {
  background-color: #111;
  width: 50px;
  height: 26px;
  border-radius: 50px;
  position: relative;
  padding: 5px;
  cursor: pointer;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.fa-moon {color: #f1c40f;}

.fa-sun {color: #f39c12;}

.checkbox-label .ball {
  background-color: #fff;
  width: 22px;
  height: 22px;
  position: absolute;
  left: 2px;
  top: 2px;
  border-radius: 50%;
  transition: transform 0.2s linear;
}

.checkbox:checked + .checkbox-label .ball {
  transform: translateX(24px);
}