space-evenly: 均匀排列每个元素,每个元素之间的间隔相等
space-between: 均匀排列每个元素,首个元素放置于起点末尾元素放置于终点

.box {
  display: flex;
  // justify-content: space-evenly;
  justify-content: space-between;
  &:before,
  &:after {
      content: "";
      display: block;
  }