html, body { height: 100%; width: 100%; }

body {
    background: url("../img/background.png") no-repeat;
    background-size: cover;
    width: 100%;
    height: 100%;
}

.login-box {
    width: 100%;
    max-width: 500px;
    background: #fff;
    padding-bottom: 10px;
    position: absolute;
    top: 50%;
    left: 50%;
    margin-left: -250px;
    margin-top: -160px;
    border-radius:10px;
}
.form-group{
    overflow: hidden;
}
@media (max-width: 500px) {
    .login-box {
        left: 0;
        margin-left:0;
    }
}