gtk2/demos/gtk-demo/fancy.css

GtkButton#fancy {
  font-weight: bold;
  background-image: linear-gradient(135deg, yellow, blue);
  border-radius: 20px;
  color: white;
}

GtkButton#fancy:hover {
  font-weight: bold;
  background-image: linear-gradient(135deg, blue, yellow);
  border-radius: 20px;
  color: white;
}

GtkButton#fancy:active {
  font-weight: bold;
  background-image: linear-gradient(yellow, yellow);
  border-radius: 20px;
  color: black;
}

GtkButton#fancy * {
  color: inherit;
}