Template:PosterGalleryCSS.css

/* Card frame */
.ksi-posters .gallerybox .thumb {
  border: 1px solid #777;
  background: #111;
  padding: 6px;
  box-shadow: 0 1px 4px rgba(0,0,0,.4);
}
.ksi-posters .thumb img { display:block; width:100%; height:auto; }

/* Make ONLY non-link caption text white (links keep site color) */
.mw-parser-output ul.gallery.ksi-posters li.gallerybox .gallerytext :not(a),
.mw-parser-output ul.gallery.ksi-posters li.gallerybox .gallerytext :not(a) * {
  color: #fff !important;
}

/* Optional: caption layout */
.ksi-posters .gallerytext {
  font-size: 90%;
  padding-top: 4px;
  text-align: center;
}