Each s2Member Pro Form includes built-in CSS class names that allow you to apply styles across all Pro-Form variations at once (e.g., checkout, registration, cancellation, billing modifications, specific post/page access, etc). Or, you can use class names that are specific to certain types of Pro-Forms, or even specific to certain sections within a Pro-Form.

The easiest way to find these class names and to customize your Pro-Forms with custom CSS that takes advantage of them; is to open a page containing a Pro-Form in a browser that supports a Web Developer Console (e.g., Chrome, Firefox). The Developer Console makes it easy for you to fiddle with custom CSS and see immediate results. See: https://developer.chrome.com/devtools/docs/elements-styles

When you have things just the way you like, take the modifications that you've made and add those to the style.css in your WordPress theme of choice. See: /wp-content/themes/[YOUR THEME]/style.css

See also: Customizing Pro-Forms