To change the background colour of the title and caption, edit this section of CSS:

/*Position the title and set the background gradient*/

.ds-advent-3 .et_pb_gallery_title {
    bottom: 50%;
    background: rgba(0, 0, 0, 0.5);
    background-image: -webkit-gradient(linear, left top, left bottom, from(rgba(0, 0, 0, 0.3)), color-stop(60%, transparent), to(rgba(0, 0, 0, 0.7)));
    background-image: linear-gradient(to bottom, rgba(0, 0, 0, 0.3) 0%, transparent 60%, rgba(0, 0, 0, 0.7) 100%);
}


/*Position the caption and set the background gradient*/

.ds-advent-3 .et_pb_gallery_caption {
    top: 50%;
    background: rgba(255, 255, 255, 0.5);
    background-image: -webkit-gradient(linear, left bottom, left top, from(rgba(255, 255, 255, 0.3)), color-stop(60%, transparent), to(rgba(255, 255, 255, 0.7)));
    background-image: linear-gradient(to top, rgba(255, 255, 255, 0.3) 0%, transparent 60%, rgba(255, 255, 255, 0.7) 100%);
}

To change the colour of the dividing line, edit this section of CSS:

/*Add the dividing line and set its animation*/

.ds-advent-3 .et_pb_gallery_item::before {
    position: absolute;
    z-index: 1;
    top: 50%;
    right: 50%;
    left: 50%;
    height: 2px;
    content: '';
    opacity: 0;
    background-color: #77B5D4;
    -webkit-transition: all 0.4s;
    transition: all 0.4s;
    -webkit-transition-delay: 0.6s;
    transition-delay: 0.6s;
}

The title and caption font colour, size etc. can be edited within the module settings.