如何修改vant的less样式变量

import ‘vant/lib/index.css';

import ‘vant/lib/toast/style/less';

//toast
@toast-background-color:blue;
@action-sheet-max-height:80%!important;

const path = require('path')
module.exports = {
    css:{
        loaderOptions:{
            less:{
                modifyVars:{
                    //通过本地less文件覆盖变量
                    // true; @import "your-less-file-path.less";
                    hack:`true;@import "${path.resolve(__dirname,'./src/common/cover_vant.less')}";`
                }
            }
        }
    }
}

.van-dropdown-menu__bar {
   background: #6609f2 !important;
   box-shadow: none !important;
 }
.van-dropdown-menu__title,
.van-dropdown-menu__title--active {
   color: #fff !important;
 }

<style lang='less'>
.van-dropdown-menu__bar {
  background: #6609f2 !important;
  box-shadow: none !important;
}
.van-dropdown-menu__title,
.van-dropdown-menu__title--active {
  color: #fff !important;
}
</style>

        <van-dropdown-menu class="btn">
          <van-dropdown-item v-model="valueCity" :options="optionCity" @change="changeCity" />
          <van-dropdown-item v-model="valuePrice" :options="optionPrice" @change="changeDropdown" />
          <van-dropdown-item v-model="valueYear" :options="optionYear" @change="changeYear" />
        </van-dropdown-menu>

<style lang="less" >
.btn > .van-dropdown-menu__bar {
  background: #fff !important;
}
.btn .van-dropdown-menu__title {
  color: #333 !important;
}
</style>