gtk/tests/reftests/unresolvable.css

GtkWindow {
  engine: none;
  background-image: none;
  background-color: @doesntexist;
  box-shadow: @nonexistant 5 5;
  border-image: -gtk-gradient(linear,
                              left top, left bottom,
                              from(@notaavailable),
                              to(@whatever)) 2;
}

#reference {
  background-color: pink;
  box-shadow: none;
  border-image: none;
}