You can edit the unsubscribe page's default content section in the WYSIWIG editor under Settings>Defaults>Unsubscribe>Unsubscribe options. 

However, the stylesheet for the unsubscribe page is not customizable at the moment. Furthermore, the default text that reads either the organization name, the unsubscribe button and the email address form are also not customizable through the theme file. You can update organization name in your nation settings if need be.

If you need to update preferences, you can edit subscription options under Settings>Defaults>Unsubscribe>Unsubscribe options. 

http://nationbuilder.com/how_to_use_the_unsubscribe_page

http://nationbuilder.com/ericdemersmtl/customizing_the_unsubscribe_page_to_adapt_to_multiple_langages