Add currency symbol in custom amount field

style.css

.custom-donation-amount-wrapper::before {
  content: "$";
  display: block;
  position: absolute;
  bottom: .5em;
  left: 1em;
  color: #bbb;
  padding: 6px 0 6px 7px;
  font-size: 1em;
}

.custom-donation-amount-wrapper label {
  display: block;
}

.charitable-donation-form .custom-donation-input {
  text-indent: 7px;
}