Server-side Google Apps Script doesn't do anything directly to style HTML output on the client side.
Option 1: Include <style> tags in your HTML file (not in GS).
<link rel="stylesheet" href="https://ssl.gstatic.com/docs/script/css/add-ons1.css">
<style>
div {
fontFamily:"Courier New";
}
</style>
...
Option 2: Inline style, again in your HTML.
<link rel="stylesheet" href="https://ssl.gstatic.com/docs/script/css/add-ons1.css">
<div style='fontFamily:"Courier New";'>
...
</div>
Option 3: Use a separate HTML file for your css, which you import into your HTML. This is demonstrated in the Best Practices documentation, under Separate HTML, CSS, and JavaScript. Since the css isn't being linked as a stylesheet, but instead is imported HTML, you need to include the <style> tags.