.leaflet-control-fullscreen a {
    background: #fff url(fullscreen.png) no-repeat 0 0;
    background-size: 26px 52px;
}
.leaflet-fullscreen-on .leaflet-control-fullscreen a {
    background-position: 0 -26px;
}


@media (-webkit-min-device-pixel-ratio:2), (min-resolution:192dpi) {
    .leaflet-control-fullscreen a {
        background-image:url(fullscreen@2x.png);
    }
}