footer {
    background-color: black;
    position: absolute;
    bottom: 0;
    width: 100%;
    height: 40px;
    color: white;
}

@media screen and (max-width:640px) {
    /* reglas CSS */
}

@media screen and (max-width:1024px) and (min-width:640px) {
    /* reglas CSS */
}

@media screen and (min-width:1024px) {
    /* reglas CSS */
}

@media Screen and (max-width:640px)and (min-width:360px) {}