Let your website's stylesheet do the work
Start with this option. Add the form to your website as-is without any customization. Often the main stylesheets of websites include settings to update the form automatically. In this case additional customization isn't required.
Customize styles in the form editor
If you need to customize styles you can do so from the form editor from the Styles navigation.
Use custom CSS
Formatting tips for forms displayed as a landing page (not embedded on web pages)
Related articles about working with forms.