html,
body {
  height: 100%;
}

main {
  flex: 1 0 auto;
background: url(../images/y-so-serious-white.png) fixed;
}
.gallery{
    margin-bottom: 10px;
}
.gallery .card-body {
    position:absolute;
    bottom: 50px;
    width: 100%;
    background-color: aqua;
    text-align: center; 
    color: white;
}

/* // Medium devices (tablets, 768px and up) */
@media (min-width: 768px) { }

/* // Large devices (desktops, 992px and up) */
@media (min-width: 992px) { }