<div class="element"></div>

.element {
  width: 100%;
  height: 100%;
  animation: pulse 5s infinite;
}

@keyframes pulse {
  0% {
    background-color: #001F3F;
  }
  100% {
    background-color: #FF4136;
  }
}

html,
body {
  height: 100%;
}