.theme-switcher {
  position: relative;
  display: inline-block;
  width: 50px;
  height: 24px;
}
.theme-switcher input { display:none; }
.slider {
  position: absolute;
  cursor: pointer;
  background-color: #ccc;
  transition: 0.4s;
  border-radius: 24px;
  top: 0; left: 0; right: 0; bottom: 0;
}
.slider:before {
  position: absolute;
  content: "";
  height: 16px; width: 16px;
  left: 4px; bottom: 4px;
  background-color: white;
  transition: 0.4s;
  border-radius: 50%;
}
input:checked + .slider {
  background-color: #2196F3;
}
input:checked + .slider:before {
  transform: translateX(26px);
}