How to add styles in existing function?

Here’s couple of examples on how to add styling to your code.

// Method 1. define classes in another variable, then do styling in styles.css
$classes="another-class";

$content .= "
<div id=\"my-id\"class=\"my-class\"> // Method 2. use backslashes to escape double quotes and add id and classes as needed, then do styling in styles.css
   <h3 class=\"{$classes}\">stuff</h3> // Method 1. interpolate classes where needed
   <div style=\"background-color: blue; color: #fff;\">{$additional_stuff}</div> // Method 3. use backslashes to escape double quotes and add inline styles
</div>";

p.s. do remove the comments from the example code before copy-pasting $content to production.