dark-mode.css

@import url("https://fonts.googleapis.com/css2?family=Poppins:wght@300;400;500;600;700&display=swap");

:root {
  --bg-color: #fff;
  --text-color: #121212;
  --trans-time: 0.4s;
}

* {
  font-family: "Poppins", sans-serif;
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  color: var(--text-color);
  background-color: var(--bg-color);
  transition: background-color var(--trans-time) ease-in-out, color var(--trans-time) ease-in-out;
  min-height: 100vh;
}

main {
  width: 500px;
  text-align: center;
}

div {
  display: flex;
  justify-content: space-evenly;
  align-items: center;
  width: 100%;
}

/* Moving the checkbox out of the page */
input[type="checkbox"]#dark-mode {
  position: absolute;
  left: -9000px;
  top: -9000px;
}

input[type="checkbox"]#dark-mode + label {
  position: relative;
  display: flex;
  align-items: center;
  cursor: pointer;
}

input[type="checkbox"]#dark-mode + label::before {
  content: "";
  width: 50px;
  height: 10px;
  background-color: #ddd;
  border-radius: 20px;
}

input[type="checkbox"]#dark-mode + label::after {
  position: absolute;
  left: 0;
  content: "";
  width: 20px;
  height: 20px;
  background-color: var(--text-color);
  border-radius: 50%;
  transition: left var(--trans-time) ease-in-out, background-color var(--trans-time) ease-in-out;
}

input[type="checkbox"]#dark-mode:checked + label::after {
  left: calc(50px - 20px); /* 50px is the width of the label */
                                 /* 20px is width of slider */
  /* transform: translateX(30px); */ /* This does the same thing */
  background-color: var(--text-color);
}