<button class="radikalwp-btn-download"><span>دانلود فایل<//span><//button>

.radikalwp-btn-download {
  position: relative;
  padding: 14px 34px;
  font-family: inherit;
  font-size: 16px;
  font-weight: 700;
  color: #fbbf24;
  background: transparent;
  border: 2px solid #fbbf24;
  border-radius: 12px;
  cursor: pointer;
  overflow: hidden;
  z-index: 1;
  transition: color .35s ease;
}
.radikalwp-btn-download::after {
  content: '';
  position: absolute;
  bottom: 0;
  left: 0;
  width: 100%;
  height: 0%;
  background: #fbbf24;
  transition: height .35s ease;
  z-index: -1;
}
.radikalwp-btn-download:hover {
  color: #1f1300;
}
.radikalwp-btn-download:hover::after { height: 100%; }