@media screen and (max-width: 640px) {
    .blocConnexion {
        width: 100%;
        background: white;
        height: 100%;
        margin-right: auto;
        margin-left: auto;
    }
    .bottomConnexion {
        position: static;
        bottom: 0;
        width: 100%;
        padding: 8px 0 8px 0;
        background-color: #3e75c1;
    }
}

@media screen and (min-width: 640px) {
    .blocConnexion {
        width: 39%;
        background: white;
        height: 100%;
        margin-right: auto;
        margin-left: auto;
    }
    .bottomConnexion {
        position: absolute;
        bottom: 0;
        width: 39%;
        padding: 8px 0 8px 0;
        background-color: #3e75c1;
    }
}