.whatsapp-fab {
  position: fixed; z-index: 95; right: 20px; bottom: 20px;
  width: 58px; height: 58px; border-radius: 50%;
  background: #25d366; color: #fff;
  display: grid; place-items: center;
}
.whatsapp-fab svg { width: 34px; height: 34px; }
@media (max-width: 560px) {
  .whatsapp-fab { width: 54px; height: 54px; right: 16px; bottom: 16px; }
  .whatsapp-fab svg { width: 30px; height: 30px; }
}