Gravity Forms are structured so that every element can be targeted and manipulated via CSS. Most elements share reusable class names to affect styling, and many elements have unique ID's that you can use to target specific elements within the form. By using CSS inheritance, you can effectively style every element in your form.
All element ID's are based on the unique form ID and then a field ID. For example: #gform_wrapper_xx where "xx" is the form ID or #field_xx_yyy where "xx" is the form ID and "yyy" is the field ID.