img.thumb { width: 10em; }
.pswp__custom-caption {
    background: rgba(75, 150, 75, 0.75);
    font-size: 16px;
    color: #fff;
    width: calc(100% - 32px);
    max-width: 400px;
    padding: 2px 8px;
    border-radius: 4px;
    position: absolute;
    left: 50%;
    bottom: 16px;
    transform: translateX(-50%);
}
.center {
  text-align: center;
}