body.login_admin {
    background: linear-gradient(to right, #0d1b2a, #1b263b, #274472, #1e3a8a);
    -webkit-background-size: 400% 400%;
    -moz-background-size: 400% 400%;
    -o-background-size: 400% 400%;
    background-size: 400% 400%;
    animation: gradientBG 15s ease infinite;
}

@keyframes gradientBG {
    0% { background-position: 0% 50%; }
    50% { background-position: 100% 50%; }
    100% { background-position: 0% 50%; }
}