HTMLify

* {
  border: 0;
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

:root {
  --hue: 223;
  --primary: hsl(var(--hue), 90%, 50%);
  --trans-dur: 0.6s;
  --trans-timing: cubic-bezier(0.65, 0, 0.35, 1);
  font-size: calc(60px + (120 - 60) * (100vw - 320px) / (2560 - 320));
}

body,
input {
  font: 1em/1.5 sans-serif;
}

body {
  background-color: hsl(var(--hue), 10%, 90%);
  color: hsl(var(--hue), 10%, 10%);
  display: flex;
  height: 100vh;
  transition:
    background-color var(--trans-dur),
    color var(--trans-dur);
}

body:has(.switch__input:checked) {
  background-color: hsl(var(--hue), 10%, 20%);
  color: hsl(var(--hue), 10%, 90%);
}

.switch,
.switch__input {
  display: block;
  -webkit-tap-highlight-color: transparent;
  cursor: pointer;
}

.switch {
  margin: auto;
  position: relative;
  user-select: none;
}

.switch__icon {
  color: hsla(var(--hue), 10%, 80%);
  pointer-events: none;
  position: absolute;
  inset: 0.375em 0 0 0.375em;
  width: 0.75em;
  height: 0.75em;
  transition:
    color var(--trans-dur),
    transform var(--trans-dur) var(--trans-timing);
}

.switch__icon:nth-of-type(2) {
  right: 0.375em;
  left: auto;
}

.switch__inner,
.switch__inner-icons {
  border-radius: 0.5em;
  display: block;
  overflow: hidden;
  position: absolute;
  inset: 0.25em 0 0 0.25em;
  width: 2.25em;
  height: 1em;
}

.switch__inner::before,
.switch__inner-icons {
  transition: transform var(--trans-dur) var(--trans-timing);
  transform: translateX(-1.25em);
}

.switch__inner::before {
  background-color: var(--primary);
  border-radius: inherit;
  content: "";
  display: block;
  width: 100%;
  height: 100%;
}

.switch__inner-icons {
  pointer-events: none;
}

.switch__inner-icons .switch__icon {
  color: hsl(0, 0%, 100%);
  inset: 0.125em 0 0 0.125em;
  transform: translateX(1.25em);
}

.switch__inner-icons .switch__icon:nth-child(2) {
  right: 0.125em;
  left: auto;
}

.switch__input {
  background-color: hsl(0, 0%, 100%);
  border-radius: 0.75em;
  box-shadow:
    0 0 0 0.0625em hsla(var(--hue), 90%, 50%, 0),
    0 0.125em 0.5em hsla(var(--hue), 10%, 10%, 0.1);
  outline: transparent;
  width: 2.75em;
  height: 1.5em;
  appearance: none;
  transition:
    background-color var(--trans-dur),
    box-shadow var(--trans-dur);
}

.switch__input:checked {
  background-color: hsl(var(--hue), 10%, 10%);
}

.switch__input:checked~.switch__icon {
  color: hsla(var(--hue), 10%, 40%);
}

.switch__input:checked~.switch__inner::before,
.switch__input:checked~.switch__inner-icons {
  transform: translateX(1.25em);
}

.switch__input:not(:checked)~.switch__icon:first-of-type,
.switch__input:checked~.switch__icon:nth-of-type(2) {
  transform: rotate(360deg);
}

.switch__input:checked~.switch__inner-icons .switch__icon:first-of-type {
  transform: translateX(-1.25em) rotate(-360deg);
}

.switch__input:checked~.switch__inner-icons .switch__icon:nth-of-type(2) {
  transform: translateX(-1.25em) rotate(360deg);
}

.switch__input:focus-visible {
  box-shadow:
    0 0 0 0.0625em hsla(var(--hue), 90%, 50%, 1),
    0 0.125em 0.5em hsla(var(--hue), 10%, 10%, 0.1);
}