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@media (max-width:499px) {
  .page-index .product,
  .page-search .product {
      width: 100%;
      min-width: 250px;
  }

  .page-index .product-miniature .thumbnail-container,
  .page-search .product-miniature .thumbnail-container {
      width: 100% !important;
  }
}

The problem is that you are editing the styles directly in theme.css. You can tell by the fact that your css is decompressed.

@media (max-width:499px) {
  .page-index .product,
  .page-search .product {
      width: 100%;
      min-width: 250px;
  }

  .page-index .product-miniature .thumbnail-container,
  .page-search .product-miniature .thumbnail-container {
      width: 100% !important;
  }
}