title: Nuxt Build Doesn't Include node_modules CSS
body: <p>Sorry I'm quite new to Nuxt &amp; Webpack.  I have a Nuxt project in which everything works fine running yarn dev.  But when I build using yarn build -> yarn generate, the .css files I reference in the node_modules directory (dropzone.css, flatpickr.css etc) are never included and break the site. I've tried everything but can't figure out what I'm doing wrong. Can someone point me in the right direction?  Currently running Nuxt v2.11.0 with Tailwind.css.  This is my nuxt.config.js file which is mostly boilerplate - </p>
  
  <pre><code>require('dotenv').config()
  
  export default {
      env: {
          baseUrl: process.env.BASE_URL || '/',
          apiUrl: process.env.API_URL
      },
      mode: 'universal',
      css: [
          '@/assets/css/tailwind.css',
          '@/assets/fonts/caslongraphique/webfont.css',
          '@/assets/fonts/turbinadobolddry/font.css',
          { src: 'nuxt-dropzone/dropzone.css', lang: 'css' },
          { src: 'vue-agile/dist/VueAgile.css', lang: 'css' },
          { src: 'flatpickr/dist/flatpickr.css', lang: 'css' },
          { src: 'flatpickr/dist/themes/airbnb.css', lang: 'css' }
      ],
      /*
      ** Plugins to load before mounting the App
      */
      plugins: [
          { src: '~plugins/helpers' },
          { src: '~plugins/vue-moment' },
          { src: '~plugins/vue-agile', mode: 'client' },
          { src: '~plugins/eventBus', mode: 'client' },
          { src: '~plugins/axios', mode: 'client' },
          { src: '~plugins/vuex-persist', mode: 'client' }
      ],
      buildModules: [
          // Doc: https://github.com/nuxt-community/nuxt-tailwindcss
          '@nuxtjs/tailwindcss',
      ],
      modules: [
          '@nuxtjs/axios',
          '@nuxtjs/dotenv',
          // 'nuxt-client-init-module'
      ],
      build: {
          /*
          ** You can extend webpack config here
          */
          extend (config, ctx) {
          },
          postcss: {
              plugins: {
                  // Disable a plugin by passing false as value
                  'postcss-url': false,
                  'postcss-nested': {},
                  'autoprefixer': true
              },
              preset: {
                  // Change the postcss-preset-env settings
                  autoprefixer: {
                      grid: true
                  }
              }
          },
      }
  }
  </code></pre>