Gallery: Downloadable icon over image on hover

To add a download icon on hover gallery image, like this.

#1. First, use this code to Custom CSS

/* gallery download icon */
.download-icon {
    position: absolute;
    z-index: 999999;
    cursor: pointer;
    top: 50%;
    left: 50%;
    transform: translate(-50%,-50%);
    display: none;
}
figure:hover .download-icon {
  display: block;
}