my.css

background: linear-gradient(270deg, #f877ff, #77ffee);
background-size: 400% 400%;
-webkit-animation: AnimationName 30s ease infinite;
-moz-animation: AnimationName 30s ease infinite;
-o-animation: AnimationName 30s ease infinite;
animation: AnimationName 30s ease infinite;
@-webkit-keyframes AnimationName {
    0%{background-position:0% 50%}
    50%{background-position:100% 50%}
    100%{background-position:0% 50%}