<button class="radikalwp-btn-fill"><span>ثبت‌نام<//span><//button>

.radikalwp-btn-fill {
  position: relative;
  padding: 13px 32px;
  font-size: 16px;
  font-family: inherit;
  color: #16a34a;
  background: transparent;
  border: 2px solid #16a34a;
  border-radius: 12px;
  cursor: pointer;
  overflow: hidden;
  z-index: 0;
  transition: color .35s ease;
}
.radikalwp-btn-fill::before {
  content: '';
  position: absolute;
  left: 0; right: 0; bottom: 0;
  height: 0;
  background: #16a34a;
  transition: height .35s ease;
  z-index: -1;
}
.radikalwp-btn-fill:hover { color: #fff; }
.radikalwp-btn-fill:hover::before { height: 100%; }