.leaflet-control-fullscreen-button {
  background-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABoAAAAaCAQAAAADQ4RFAAAAUklEQVR4AWOgGhgF/8HwPIrYeYgoIU0OKGIOxGm6jyJ2n5Cm8wwOQEUGKGIGQBEHoAwB0AA0FwEbSAgOBBwWmggHBOVBTjhyKU9GhBMslcAoAABPu2Hh6JIyeQAAAABJRU5ErkJggg==);
}

body { margin:0; padding:0; }