bartik_slick_fix.css in Thunder 6.2.x

.slick__slide .field.field--type-image {
  margin: 0;
}

/* For slick galleries using blazy, with non-responsive image styles, this can help, if gallery takes too much vertical space.
Background: Not loaded images have a 1x1 placeholder. This means, if the width is set to 900px, It will take 900px in height,
 disregarding the set height. */
/*
.slick__slide.slide--loading .blazy {
    height: 0;
}
*/