[2069] how to customizing variables bootstrap4

https://v4-alpha.getbootstrap.com/getting-started/options/

nuxt.config.js

    css: [
      'bootstrap/scss/bootstrap.scss',
      '~/styles/index.scss',
      'bootstrap-vue/dist/bootstrap-vue.css',
    ],
    plugins: [
      '~/plugins/bootstrap-vue.js'
    ],

and how to add bootstrap-loader?

require('bootstrap-loader?bootstrapPath=/path/to/bootstrap');

// or

entry: [ 'bootstrap-loader?bootstrapPath=/path/to/bootstrap', './app' ]