li.slide-item.slick-slide {
  max-width: 250px;
  max-height: 140px;
}
.gallery-inner {
  width: 250px;
}
.thumbnail-item {
  width: 50px;
  height: 50px;
}
.thumbnail-item img {
  max-height: 50px;
  max-width: 50px;
}

ul.slick-initialized.slick-slider li.slide-item.slick-slide img {
  width: initial;
  max-width: 250px;
  height: auto;
  max-height: 100%;
}
@media screen and (max-width: 480px) {
  /* 480px以下に適用されるCSS（スマホ用） */
  .gallery-inner {
    width: 100%;
  }
  li.slide-item.slick-slide {
    height: auto;
    max-height: 300px;
    max-width: 100%;
  }
  .thumbnail-item img {
    max-height: 80px;
    max-width: 80px;
  }
  .thumbnail-item {
    width: 20% !important;
  }
  #slider {
    height: auto;
    max-height: 200px;
  }
}