James Revillini on "[Plugin: Visual Form Builder] using form id's to edit specific forms with css"

form.vfb-form-4 { /* address form id 4 */ }

form.vfb-form-4 fieldset { /* address fieldsets in form id 4 */ }

form.vfb-form-4 input.vfb-text { /* address text input in form id 4 */ }

Use a browser inspector as Matt recommends on his site: http://matthewmuro.com/2012/11/15/visual-form-builder-pro-customizing-the-form-design/