If you want to change submit button, add name of button to form class.

Web-form button

If you want to change open-popup button, specify class attribute to it.

Web-form button

Example of buttons:

multiform-button-1 multiform-button-2 multiform-button-3 multiform-button-4 multiform-button-5 multiform-button-6 multiform-button-7 multiform-button-8 multiform-button-9 multiform-button-10 multiform-button-11 multiform-button-12 multiform-button-13
multiform-button-1-flat multiform-button-2-flat multiform-button-3-flat multiform-button-4-flat multiform-button-5-flat multiform-button-6-flat multiform-button-7-flat multiform-button-8-flat multiform-button-9-flat multiform-button-10-flat multiform-button-11-flat multiform-button-12-flat multiform-button-13-flat