Removing the shadow from a button

Use border-style:

.signup-success input[type="submit"],
.signup-success input[type="submit"]:active,
.signup-success input[type="submit"]:focus {
  width: 80%;
  background: transparent;
  color: #00AA66;
  border-color: #00AA66;
  border-style: solid;
}

or combined version (border-style, border-width and border-color in one):

border: 2px solid #00AA66;