.hide {
    display:none;
}

#backToTop {
  position: fixed;
  bottom: 40px;
  right: 40px;
  z-index: 999;
  background: #fff;             /* 白底 */
  border: none;
  border-radius: 50%;           /* 圆形按钮 */
  box-shadow: 0 2px 8px rgba(0,0,0,0.15); /* 柔和阴影 */
  width: 48px;
  height: 48px;
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.3s ease, transform 0.3s ease, box-shadow 0.3s ease;
  padding: 0;
}

#backToTop.show {
  opacity: 1;
  pointer-events: auto;
}

#backToTop:hover {
  transform: translateY(-4px);
  box-shadow: 0 4px 12px rgba(0,0,0,0.2);
}

#backToTop img {
  width: 24px;    /* 图标大小 */
  height: 24px;
  filter: brightness(0); /* 如果你是黑箭头，就保留这句 */
}

#contacts {
  position: relative;              /* 必须有定位，伪元素才能绝对定位在里面 */
  background-size: cover;
  background-position: top;
}

#contacts::before {
  content: "";
  position: absolute;
  inset: 0;                        /* 顶部、底部、左右都铺满 */
  background: rgba(0, 0, 0, 0.3);   /* 半透明黑底，可调透明度 0.3~0.6 */
  z-index: 0;                       /* 放在最底层，低于文字 */
}

/* 保证文字等内容在遮罩之上 */
#contacts > * {
  position: relative;
  z-index: 1;
}

