.red{
position:absolute;
top:0;
z-index:-1;
width:100%;
height:100%;
opacity:0.5;
background: radial-gradient(red, green, blue);
animation:myframes 3s linear infinite alternate;

}
@keyframes myframes{
   0%{background: radial-gradient(white, green, yellow);}
   50%{background: radial-gradient(green,white,yellow);}
   100%{background: radial-gradient(white,yellow, green);}
}