First if you want to propose a service to your customer to create website based on "joomla template" then you can get inspired of tutorial video 13
www.jms2win.com/en/tutorial#jms12x-13
In this case, you will have to create several slave site that will be setup with the appropriate "Joomla template" or provide them a set of "Joomla template" from which they will be able to select a "default" template in the back-end.
Each website content will be a clone, a copy of the website that you will create as "website template".
The content of the website will not be shared.
If you want to share some content, this is possible for a limited number of extensions.
See the list of extension in the JMS Tool - help menu or here
www.jms2win.com/component/content/article/135
Share a whole website with exactly the same content for all the website and only another "default joomla template" is not immediately possible.
If you share the whole website, you can just replace the content of the "joomla template" directory with another template content so that you keep the same name but with another look.