background: linear-gradient(309deg, #b954fd, #339aff, #28f7f7);
background-size: 600% 600%;

-webkit-animation: musicbox 15s ease infinite;
-moz-animation: musicbox 15s ease infinite;
-o-animation: musicbox 15s ease infinite;
animation: musicbox 15s ease infinite;

@-webkit-keyframes musicbox {
    0%{background-position:0% 32%}
    50%{background-position:100% 69%}
    100%{background-position:0% 32%}
}
@-moz-keyframes musicbox {
    0%{background-position:0% 32%}
    50%{background-position:100% 69%}
    100%{background-position:0% 32%}
}
@-o-keyframes musicbox {
    0%{background-position:0% 32%}
    50%{background-position:100% 69%}
    100%{background-position:0% 32%}
}
@keyframes musicbox {
    0%{background-position:0% 32%}
    50%{background-position:100% 69%}
    100%{background-position:0% 32%}
}