微信小程序入门中scroll-view那块颜色不显示问题

在学习微信小程序开发中，到了scroll-view这一块的时候，为了给我们更好的讲解。给我们提供了测试代码。
.xwml文件

<view class="section">
  <view class="section__title">vertical scroll</view>
  <scroll-view
    scroll-y
    style="height: 200px;"
    bindscrolltoupper="upper"
    bindscrolltolower="lower"
    bindscroll="scroll"
    scroll-into-view="{{toView}}"
    scroll-top="{{scrollTop}}"
  >
    <view id="green" class="scroll-view-item bc_green"></view>
    <view id="red" class="scroll-view-item bc_red"></view>
    <view id="yellow" class="scroll-view-item bc_yellow"></view>
    <view id="blue" class="scroll-view-item bc_blue"></view>
  </scroll-view>

  <view class="btn-area">
    <button size="mini" bindtap="tap">click me to scroll into view</button>
    <button size="mini" bindtap="tapMove">click me to scroll</button>
  </view>
</view>
<view class="section section_gap">
  <view class="section__title">horizontal scroll</view>
  <scroll-view class="scroll-view_H" scroll-x style="width: 100%">
    <view id="green" class="scroll-view-item_H bc_green"></view>
    <view id="red" class="scroll-view-item_H bc_red"></view>
    <view id="yellow" class="scroll-view-item_H bc_yellow"></view>
    <view id="blue" class="scroll-view-item_H bc_blue"></view>
  </scroll-view>
</view>

.js文件

const order = ['red', 'yellow', 'blue', 'green', 'red']
Page({
  data: {
    toView: 'red',
    scrollTop: 100
  },
  upper(e) {
    console.log(e)
  },
  lower(e) {
    console.log(e)
  },
  scroll(e) {
    console.log(e)
  },
  tap(e) {
    for (let i = 0; i < order.length; ++i) {
      if (order[i] === this.data.toView) {
        this.setData({
          toView: order[i + 1]
        })
        break
      }
    }
  },
  tapMove(e) {
    this.setData({
      scrollTop: this.data.scrollTop + 10
    })
  }
})

想要我们显示的效果如下

但是复制代码以后没有显示想要的颜色
需要在wxss里面设置对应得颜色

.bc_green {
  background-color: green;
}

.bc_red {
  background-color: red;
}

.bc_yellow {
  background-color: yellow;
}

.bc_blue {
  background-color: blue;
}