swap.css

.leaflet-sbs-swap {
    position: absolute;
    -webkit-appearance: none;
    margin: 0;
    padding: 0;
    background:  #2C405A;
    height: 30px;
    width: 30px;
    border-radius: 50%;
    cursor: pointer;
    pointer-events: auto;
    border: 2px solid #2C405A;
    color: white;
    background-image: url("data:image/svg+xml,<svg width='16' height='12' xmlns='http://www.w3.org/2000/svg'><path d='M3.825 5.166L.5 8.499l3.325 3.334v-2.5h5.842V7.666H3.825v-2.5zM15.5 3.499L12.175.166v2.5H6.333v1.667h5.842v2.5L15.5 3.499z' fill='white'/></svg>");
    background-position: 50% 50%;
    background-repeat: no-repeat;
    background-size: 16px 12px;
    z-index: 999;
}

.leaflet-sbs-swap:focus {
    outline: none;
    box-shadow: 0px 0px 4px white;
}

.leaflet-sbs-swap::before,
.leaflet-sbs-swap::after {
    color: white;
    position: absolute;
    font-size: 8px;
    font-weight: bolder;
}

.leaflet-sbs-swap::before {
    content: 'A';
    content: attr(data-left);
    top: calc(50% - 11px);
    left: calc(50% - 11px);
}

.leaflet-sbs-swap::after {
    content: 'B';
    content: attr(data-right);
    top: calc(50%);
    left: calc(50% + 4px);
}