Thanks for posting your question to the forum. In taking a look at the previous post that you linked to, it sounds like you're looking to use the shortcode to add a form onto your website, thenstyle that with your own CSS. This is definitely doable!
To clarify, the process of incorporating your own CSS code should not be affected by whether you decide to add a form using the shortcode or not. You should see a consistency in naming conventions in the sign up forms that are generated with the various methods of adding it which are available.
So, the approach should still be the same. Add your shortcode to your site, then add your custom CSS code to your website. If you're using a WP theme, you can even add your CSS to your theme's primary stylesheet, which is typically in the WP admin area under Appearance > Editor.
If you're looking for what elements to use in your custom CSS code, I'd recommend using your browser's view source tool or, even better, the 'inspect element' tool in your Chrome browser.
If you have any questions, just let me know!