.course-back-to-index {
  position: fixed;
  left: 18px;
  bottom: 18px;
  z-index: 9999;
  display: inline-flex;
  align-items: center;
  gap: 8px;
  min-height: 42px;
  box-sizing: border-box;
  padding: 9px 15px;
  border: 2px solid #1d2a32;
  border-radius: 999px;
  background: rgba(255, 253, 246, .94);
  color: #1d2a32;
  box-shadow: 4px 4px 0 rgba(29, 42, 50, .16);
  font: 800 14px/1.2 "PingFang SC", "Microsoft YaHei", sans-serif;
  letter-spacing: .02em;
  text-decoration: none;
  -webkit-backdrop-filter: blur(8px);
  backdrop-filter: blur(8px);
  transition: transform .18s ease, box-shadow .18s ease, background-color .18s ease;
}

.course-back-to-index::before {
  content: "←";
  display: grid;
  place-items: center;
  width: 23px;
  height: 23px;
  border-radius: 50%;
  background: #116a68;
  color: #fff;
  font-size: 16px;
  line-height: 1;
}

.course-back-to-index:hover {
  transform: translate(-2px, -2px);
  background: #fff4c9;
  box-shadow: 6px 6px 0 rgba(29, 42, 50, .16);
}

.course-back-to-index:focus-visible {
  outline: 3px solid #f4c95d;
  outline-offset: 3px;
}

@media (max-width: 700px) {
  .course-back-to-index {
    left: 10px;
    bottom: 10px;
    min-height: 40px;
    padding: 8px 12px;
    box-shadow: 3px 3px 0 rgba(29, 42, 50, .14);
  }
}

@media print {
  .course-back-to-index { display: none !important; }
}

@media (prefers-reduced-motion: reduce) {
  .course-back-to-index { transition: none; }
}