* {
  all: unset;
  color: transparent;
}

GtkBox *:nth-child(1) {
  background-color: red;
}

GtkBox *:nth-child(2) {
  background-color: orange;
}

GtkBox *:nth-child(3) {
  background-color: yellow;
}

GtkBox *:nth-child(4) {
  background-color: lime;
}

GtkBox *:nth-child(5) {
  background-color: blue;
}

GtkBox *:nth-child(6) {
  background-color: indigo;
}

GtkBox *:nth-child(7) {
  background-color: darkViolet;
}