/* Neutral helpers (dùng khi muốn viền/hover trung tính đồng bộ) */
.theme-surface { background-color: #fff; }
.theme-border  { border-color: #e5e7eb; } /* ~ tailwind neutral-200 */
.theme-border-strong { border-color: #d1d5db; } /* neutral-300 */
.theme-ring    { outline: 2px solid #9ca3af22; outline-offset: 2px; } /* subtle */

/* Thumbnail: B/W mặc định, hover → màu */
.thumb-bw {
  filter: grayscale(100%);
  transition: filter 200ms ease, transform 200ms ease;
}
.thumb-bw:hover {
  filter: none;
  transform: translateZ(0); /* hint GPU */
}