gtk/demos/gtk-demo/dnd.css

label.canvasitem {
  padding: 10px;
  margin: 1px;
}

.canvasitem.rainbow1,
image.rainbow1 {
  background: linear-gradient(140deg,red,orange,yellow,green,blue,purple);
}

.canvasitem.rainbow2,
image.rainbow2 {
  animation: rainbow2 1s infinite linear;
}

@keyframes rainbow2 {
 0% { background: linear-gradient(0deg,red,orange,yellow,green,blue,purple); }
 25% { background: linear-gradient(90deg,red,orange,yellow,green,blue,purple); }
 50% { background: linear-gradient(180deg,red,orange,yellow,green,blue,purple); }
 75% { background: linear-gradient(270deg,red,orange,yellow,green,blue,purple); }
 100% { background: linear-gradient(360deg,red,orange,yellow,green,blue,purple); }
}

.canvasitem.rainbow3,
image.rainbow3 {
  animation: rainbow3 1s infinite linear;
}

@keyframes rainbow3 {
 0% { background: linear-gradient(140deg,red,orange,yellow,green,blue,purple); }
 16.6% { background: linear-gradient(140deg,purple,red,orange,yellow,green,blue); }
 33.2% { background: linear-gradient(140deg,blue,purple,red,orange,yellow,green); }
 50% { background: linear-gradient(140deg,green,blue,purple,red,orange,yellow); }
 66.6% { background: linear-gradient(140deg,yellow,green,blue,purple,red,orange); }
 83.2% { background: linear-gradient(140deg,orange,yellow,green,blue,purple,red); }
 100% { background: linear-gradient(140deg,red,orange,yellow,green,blue,purple); }
}