.banner {
  width: 100%;
  height: 200px;
  color: white;
  text-align: center;
  display: flex;
  flex-direction: column;
  justify-content: center;
  animation: cambiarFondo 10s infinite;
}

/* Animación */
@keyframes cambiarFondo {
  0%   { background: red; }
  33%  { background: blue; }
  66%  { background: green; }
  100% { background: red; }
}