body.login_admin {
    background: linear-gradient(to right, #1C1C1C, #2E2E2E, #3A3A3A, #4B4B4B);
    -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%; }
}