<div id="rechteck"></div>

@keyframes hintergrundfarbe {
        0% {
            background-color:#F9086D;
        }
        25% {
            background-color:#222222;
        }
        50% {
            background-color:#05e4d7;
            border-radius: 120px;
            }
        75% {
            background-color:#222222;
        }
        100% {
            background-color:#F9086D;
        }
    }


#rechteck{
  width: 250px;
  height: 250px;
  animation-name: hintergrundfarbe;
  animation-duration:10s;
  animation-iteration-count:infinite;
}