select2/docs/_sass/_buttons.scss

.btn-outline-inverse {
  color: #428BCA;
  background-color: transparent;
  border-color: #428BCA;
  padding: 15px 30px;
  font-size: 20px;
  transition: all .1s ease-in-out;

  &:hover {
    color: #fff;
    border-color: #428BCA;
    background-color: #428BCA;
  }

  .fa {
    margin-right: 8px;
  }
}