Note: These selectors apply only to the Gravity Forms 2.5 theme.
Example modifying all forms
This will change the background color for all forms by using the form wrapper element
body .gform_wrapper {
background-color: yellow;
}
Example modifying one form
This will target a specific form, in this case, form ID 1, as indicated by the extended element name. Refer to this article for the structure of element names.
body #gform_wrapper_1 {
background-color: yellow;
}
Placement
You can add your Custom CSS to the Customizer (Appearance → Customize) or your child theme stylesheet.
See also: Where to Put your Custom CSS.
More Info
See this article for dozens more examples of using CSS to target your form elements.