src/scss/elements/_toggle-switch.scss

@mixin marginTransition {
  -webkit-transition: margin 0.4s, background 0.4s;
  -moz-transition: margin 0.4s, background 0.4s;
  -o-transition: margin 0.4s, background 0.4s;
  transition: margin 0.4s, background 0.4s;
}

@mixin borderRadius {
  -webkit-border-radius: 60px;
  -moz-border-radius: 60px;
  border-radius: 60px;
}

.toggle-switch {
  position: absolute;
  left: 20px;
  height: 1px;
  width: 1px;
  opacity: 0;
}

.toggle-switch + label {
  display: block;
  position: relative;
  cursor: pointer;
  outline: none;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

.toggle label {
  text-indent: -9999px;
  transform: translateY(-50%); // keeps switch centered
}

input.toggle-switch__round-flat + label {
  padding: 2px;
  width: 50px;
  height: 30px;
  background-color: $lightgrey;
  @include borderRadius;
  @include marginTransition;
}

input.toggle-switch__round-flat:focus + label {
  box-shadow: 0 0 4px 4px $darkbg__accent;
}
input.toggle-switch__round-flat + label:before, input.toggle-switch__round-flat + label:after {
  display: block;
  position: absolute;
  content: "";
}
input.toggle-switch__round-flat + label:before {
  top: 2px;
  left: 2px;
  bottom: 2px;
  right: 2px;
  background-color: $white;
  @include borderRadius;
  @include marginTransition;
}
input.toggle-switch__round-flat + label:after {
  top: 4px;
  left: 4px;
  bottom: 4px;
  width: 20px;
  background-color: $lightgrey;
  -webkit-border-radius: 52px;
  -moz-border-radius: 52px;
  -ms-border-radius: 52px;
  border-radius: 52px;
  @include marginTransition;
}
input.toggle-switch__round-flat:checked + label {
  background-color: $primebtn__bg;
}
input.toggle-switch__round-flat:checked + label:before {
  background-color: $lightbg__accent;
}
input.toggle-switch__round-flat:checked + label:after {
  margin-left: 20px;
  background-color: $primebtn__bg;
}