.btn-flat-bottom-border {
  position: relative;
  display: inline-block;
  font-weight: bold;
  padding: 7px 10px 10px 10px;
  text-decoration: none;
  color: #FFF;
  background: #a180dc;
  transition: .4s;
}

.btn-flat-bottom-border > span {
  border-bottom: solid 2px #FFF;
}

.btn-flat-bottom-border:hover {
  background: #91b5fb;
}