gtk2/tests/reftests/border-image-gradient.css

.button-image {
  engine: none;

  border-image-source: -gtk-gradient(linear,
                                     left top, left bottom,
                                     from(green),
                                     to(green));
  border-image-slice: 1;
  border-image-repeat: stretch;

  border-width: 1 2 2 1;
  border-radius: 0;
}

.button-not-image {
  border-color: green;
  border-style: solid;
  border-width: 1 2 2 1;
  border-radius: 0;
}