When it comes to customizing form styles you have three options:
OPTION 1: Let your website's stylesheet do the work
First, add the form to your website as-is without any customization. Often the main stylesheets of websites will include settings to update the form. In this case additional customization isn't required.
OPTION 2: Customize styles in the form editor
If you need to customize styles you can do so from the form editor. Click the "gear" icon then use the style controls to update the form.
OPTION 3: Use custom CSS
For advanced customization beyond what's possible from the visual editor you can use custom CSS. Read the how to style forms article for developers.
More form settings
Read more about working with forms such as styles, standard and custom fields, display options, subscriptions, etc.