<button class="radikalwp-btn-rainbow"><span class="radikalwp-btn-rainbow-text">دانلود رایگان<//span><//button>

.radikalwp-btn-rainbow {
  position: relative;
  padding: 14px 34px;
  font-family: inherit;
  border: none;
  border-radius: 12px;
  cursor: pointer;
  background: #111827;
  overflow: hidden;
}
.radikalwp-btn-rainbow::before {
  content: '';
  position: absolute;
  top: -50%;
  right: -75%;
  width: 40%;
  height: 200%;
  background: linear-gradient(90deg, transparent, rgba(255,0,150,.75), rgba(0,204,255,.75), rgba(255,220,0,.75), transparent);
  transform: rotate(25deg);
  transition: right .55s ease;
}
.radikalwp-btn-rainbow:hover::before { right: 135%; }
.radikalwp-btn-rainbow-text {
  position: relative;
  z-index: 1;
  font-size: 16px;
  font-weight: 700;
  color: #fff;
}