gtk/tests/reftests/bad-color-names.css

@define-color nonexistant @idonotexist;

@define-color self @self;

@define-color cycle @cycle1;
@define-color cycle1 @cycle2;
@define-color cycle2 @cycle1;

.background {
  background: linear-gradient(to bottom, @nonexistant, @self, @cycle, @undefinedcolor);
}