@mixin center($f: row, $a: center, $j: center) {
  display: flex;
  flex-direction: $f; // 子组件水平排列
  align-items: $a; // 子组件垂直居中对齐
  justify-content: $j; // 子组件横向居中对齐
}
.head {
  margin-top: var(--top);
  width: 100%;
  height: var(--height);
  @include center(row, center, var(--position));
  .title {
    font-size: 36rpx;
    font-weight: bold;
    color: white;
    opacity: 0.9;
  }
}