.corner {
  height: 30px;
  width: 30px;
  border-radius: 50%;
  border: 1px solid #fff;
  transform: rotate(-45deg);
  position: absolute;
  background: #1d1f20;
}

#left_top {
  top: -16px;
  left: -16px;
  border-color: transparent transparent #f1c40f transparent;
}
#right_top {
  top: -16px;
  right: -16px;
  border-color: transparent transparent transparent #f1c40f;
}
#left_bottom {
  bottom: -16px;
  left: -16px;
  border-color: transparent #f1c40f transparent transparent;
}
#right_bottom {
  bottom: -16px;
  right: -16px;
  border-color: #f1c40f transparent transparent transparent;
}