.css-selector {
    background: linear-gradient(115deg, #51ae96, #1f54d5, #bc1fd5, #eed613);
    background-size: 800% 800%;

    -webkit-animation: AnimationName 10s ease infinite;
    -moz-animation: AnimationName 10s ease infinite;
    animation: AnimationName 10s ease infinite;
}

@-webkit-keyframes AnimationName {
    0%{background-position:0% 20%}
    50%{background-position:100% 81%}
    100%{background-position:0% 20%}
}
@-moz-keyframes AnimationName {
    0%{background-position:0% 20%}
    50%{background-position:100% 81%}
    100%{background-position:0% 20%}
}
@keyframes AnimationName {
    0%{background-position:0% 20%}
    50%{background-position:100% 81%}
    100%{background-position:0% 20%}
}