Hello.
You can add CSS class to every bizform´s field, in
CMS Desk -> Tools -> BizForms -> (edit) -> Fields -> (select requested field) -> Switch to advance mode. Here you can see and set 3 properties in “Design” section: Caption style, Input style, Control CSS class. If you want to set it in your custom form layout, you can switch to source mode (using "Source" button on your CKEditor panel) and write it into html code directly.
Best Regards,
Radek Macalik