You have to import your styles before ./App import, something like that

import React from "react";
import ReactDOM from "react-dom";
import "./index.css";
import "bootstrap-css-only/css/bootstrap.min.css";
import "mdbreact/dist/css/mdb.css";
import App from "./App";
import * as serviceWorker from "./serviceWorker";

https://ibb.co/FWGTGVp

I removed your demo link because you shared a private token to our pro gitlab version, I recommend to change and disabled this token and generate a new one.

You can find an example (with all packages) here:

removed

It's just a create react app (latest version) with nothing else but the multi select. Still gives me the same result:

https://gifyu.com/image/mXnr