Меняющийся фон сайта на градиенте

Просто добавляем в css

body{
background: linear-gradient(326deg, #316155, #5fc370, #b8c35f, #c3905f);
background-size: 800% 800%;
-moz-animation: screenplug 11s ease infinite;
-o-animation: screenplug 11s ease infinite;
animation: screenplug 11s ease infinite;}
@-webkit-keyframes screenplug {
    0%{background-position:14% 0%}
    50%{background-position:87% 100%}
    100%{background-position:14% 0%}
}
@-moz-keyframes screenplug {
    0%{background-position:14% 0%}
    50%{background-position:87% 100%}
    100%{background-position:14% 0%}
}
@-o-keyframes screenplug {
    0%{background-position:14% 0%}
    50%{background-position:87% 100%}
    100%{background-position:14% 0%}
}
@keyframes screenplug {
    0%{background-position:14% 0%}
    50%{background-position:87% 100%}
    100%{background-position:14% 0%}
}

Пример тут