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.