There is no way to apply a theme to every form at once in your process, however there are some best practices to follow when using Forms, create your theme at the start and save it as a Custom Theme, then design one form then duplicate the Form, this will have the same fields and theme settings, you can then remove the fields you don't need and customise and work from there. 

You can make changes to an existing theme, by editing the existing theme and then saving it as the same name as your custom theme and this will overwrite the old Theme, and apply the changes to all the forms that the theme is applied too.