Use Bootstwatch Materia theme

// public/index.html
<link
    rel="stylesheet"
    href="https://cdn.jsdelivr.net/npm/bootswatch@4.5.2/dist/materia/bootstrap.min.css"
/>

Trying bootstrap css

// App.js
export default function App() {
  return (
    <div>
      <h1 className="display-1 bg-primary text-light p-5">App</h1>
    </div>
  );
}