Add `main.scss` as an entry point for the dev build

webpack-hot-dev-server.config.js

@@ -55,6 +55,7 @@ module.exports = require("./make-webpack-config")(rules, {
       "./src/core/index.js"
     ],
     "swagger-ui-standalone-preset": [
+      "./src/style/main.scss",
       "./src/polyfills",
       "./src/standalone/index.js",
     ]