In React you can create a 'index.css' file and import it into your application's entrypoint like this:
import './index.css'
inside this file you can add the CSS provided by @JoelBennett.
.esri-view .esri-view-surface:focus::after {
outline-color: #0000FF;
}
Alternatively, you can link the CSS-file to your primary HTML-file like this:
<!doctype html>
<html lang="en">
<head>
...
<link href="path-to-your-index.css" rel="stylesheet" />
</head>
<body>
...
</body>
</html>
whichever method you employ, you must ensure that the official Esri CSS file is imported into your project before you modify it with your own CSS.
Hope this helps! 🙂
Martin I. Bekkos
Geodata AS - Esri Distributor Norway