微信小程序实现这种效果：

<view class="topbg">
  <view class="topbg_top">
    
  </view>
  <view class="topbg_bottom">

  </view>
</view>

.topbg{width:100%;height:350rpx;text-align: center;}
.topbg_top{width:80%;height:300rpx;background-color:#fff;position: absolute;border-radius: 8rpx;left:10%;}
.topbg_bottom{width:100%;height:100rpx;background-color:rgb(41,125,96);}