ar.falsy.cat/assets/darkmode.scss

.darkmode {
  float: right;
  padding: 1em;
  min-width: 30px;
  position: relative;

  @media all and (max-width: 450px) {
    padding: 1em;
  }

  & > .toggle {
    display: none;
    box-sizing: border-box;
  }

  & svg {
    opacity: 0;
    position: absolute;
    width: 20px;
    height: 20px;
    top: calc(50% - 10px);
    margin: 0 7px;
    fill: var(--gray);
    transition: opacity 0.1s ease;
  }
}

.toggle:checked ~ label {
  & > #dayIcon {
    opacity: 0;
  }
  & > #nightIcon {
    opacity: 1;
  }
}

.toggle:not(:checked) ~ label {
  & > #dayIcon {
    opacity: 1;
  }
  & > #nightIcon {
    opacity: 0;
  }
}