renderTemplate() renders the twig file without any CSS
I sincerely appreciate the responses!
I've since resolved this issue, below is the changed code. My controller was extending the Craft web controller (\craft\web\Controller) which has its own renderTemplate() method that I switched to, which fetches the Control Panel assets I wanted for styling the page.

The template twig file remains identical as is above.

(Modified) Controller: Controller.php

class Controller extends \craft\web\Controller
{
    public function actionIndex(){

        Craft::info("hello!", __METHOD__);

        return $this-&gt;renderTemplate('plugin/done');
    }
}


This did the trick for what I wanted. Thanks again!