1. The problem occurs as follows

1. element-ui version 2.4.11 webpack ^ 3.6.0
2. use sass to add sass configuration. For more information, please see build/utils.js
3. element-ui/lib/theme-chalk/index.css there is index.css in this directory

related codes

main.js

import Vue from "vue"
import ElementUI from "element-ui"
import "element-ui/lib/theme-chalk/index.css"
Vue.use(ElementUI)

package.json

"dependencies": {
    "axios": "^0.18.0",
    "element-ui": "^2.4.11",
    "postcss": "^7.0.14",
    "vue": "^2.5.2",
    "vue-router": "^3.0.1"
  },
  "devDependencies": {
    "babel-core": "^6.22.1",
    "babel-eslint": "^8.2.1",
    "babel-helper-vue-jsx-merge-props": "^2.0.3",
    "babel-loader": "^7.1.1",
    "babel-plugin-syntax-jsx": "^6.18.0",
    "babel-plugin-transform-runtime": "^6.22.0",
    "babel-plugin-transform-vue-jsx": "^3.5.0",
    "babel-preset-env": "^1.3.2",
    "babel-preset-stage-2": "^6.22.0",
    "chalk": "^2.0.1",
    "copy-webpack-plugin": "^4.0.1",
    "css-loader": "^0.28.0",
    "eslint": "^4.15.0",
    "eslint-config-standard": "^10.2.1",
    "eslint-friendly-formatter": "^3.0.0",
    "eslint-loader": "^1.7.1",
    "eslint-plugin-import": "^2.7.0",
    "eslint-plugin-node": "^5.2.0",
    "eslint-plugin-promise": "^3.4.0",
    "eslint-plugin-standard": "^3.0.1",
    "eslint-plugin-vue": "^4.0.0",
    "extract-text-webpack-plugin": "^3.0.0",
    "file-loader": "^1.1.4",
    "friendly-errors-webpack-plugin": "^1.6.1",
    "html-webpack-plugin": "^2.30.1",
    "node-notifier": "^5.1.2",
    "node-sass": "^4.11.0",
    "optimize-css-assets-webpack-plugin": "^3.2.0",
    "ora": "^1.2.0",
    "portfinder": "^1.0.13",
    "rimraf": "^2.6.0",
    "sass": "^1.17.2",
    "sass-loader": "^7.1.0",
    "sass-resources-loader": "^2.0.0",
    "semver": "^5.3.0",
    "shelljs": "^0.7.6",
    "style-loader": "^0.23.1",
    "uglifyjs-webpack-plugin": "^1.1.1",
    "url-loader": "^0.5.8",
    "vue-loader": "^13.3.0",
    "vue-style-loader": "^3.0.1",
    "vue-template-compiler": "^2.5.2",
    "webpack": "^3.6.0",
    "webpack-bundle-analyzer": "^2.9.0",
    "webpack-dev-server": "^2.9.1",
    "webpack-merge": "^4.1.0",
    "lodash": "^4.17.11"
  },

build/utils.js

css: generateLoaders(),
    postcss: generateLoaders(),
    less: generateLoaders("less"),
    sass: generateLoaders("sass", { indentedSyntax: true }),
    scss: generateLoaders("sass").concat(
      {
        loader: "sass-resources-loader",
        options: {
          resources: path.resolve(__dirname, "../static/sass/main.scss")
        }
      }
    )

webpack.base.conf.js

 rules: [
      ...(config.dev.useEslint ? [createLintingRule()] : []),
      {
        test: /\.vue$/,
        loader: "vue-loader",
        options: vueLoaderConfig
      },
      {
        test: /\.js$/,
        loader: "babel-loader",
        include: [resolve("src"), resolve("test"), resolve("node_modules/webpack-dev-server/client")]
      },
      {
        test: /\.(png|jpe?g|gif|svg)(\?.*)?$/,
        loader: "url-loader",
        options: {
          limit: 10000,
          name: utils.assetsPath("img/[name].[hash:7].[ext]")
        }
      },
      {
        test: /\.(mp4|webm|ogg|mp3|wav|flac|aac)(\?.*)?$/,
        loader: "url-loader",
        options: {
          limit: 10000,
          name: utils.assetsPath("media/[name].[hash:7].[ext]")
        }
      },
      {
        test: /\.(woff2?|eot|ttf|otf)(\?.*)?$/,
        loader: "url-loader",
        options: {
          limit: 10000,
          name: utils.assetsPath("fonts/[name].[hash:7].[ext]")
        }
      },
      { test: /.(eot|woff|ttf)$/, loader: "url-loader" }
    ]

what result do you expect? What is the error message actually seen?