body {
    background: url(../images/img01.jpg) no-repeat center center fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
    font-family: 'Montserrat', sans-serif;
}
.cajita {
    font-weight: bold;
    color: White;
    background-color: #ffc107;
    padding: 10px;
    border-radius: 12px;
    text-align: center;
    vertical-align: middle;
    font-size: 2.5em;
    width: 60%;
    margin-top: 10%;
    margin-left: auto;
    margin-right: auto;
}
@media screen and (max-width: 768px) {
body {
    background: url(../images/img02.jpg) no-repeat center center fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}.cajita {
    font-weight: bold;
    color: White;
    background-color: #ffc107;
    padding: 10px;
    border-radius: 12px;
    text-align: center;
    vertical-align: middle;
    font-size: 1.2em;
    width: 70%;
    margin-top: 10%;
    margin-left: auto;
    margin-right: auto;    
}
}