I asked fabien a related question at the Odoo open day in Belgium this June - "how would website themes be made available for upload to their saas (no external modules allowed) , and could designers charge for them (thereby creating a marketplace for designs) ?"
He said that they were working on the idea of data-only extensions to be allowed on their saas, primarily I understood, to allow different website themes to be used on their saas (I believe there is a theme called zen that they have been working on but I've never tried to install it or activate on one of their saas instances so I don't know if this really shows the idea of a data-only module).
The question of whether a theme as a data-only module could be charged for or whether it has to be published for free should be clarified by asking odoo SA - I believe that they want to encourage theme developers (there is apparently a 'designer' tutorial in this list of documentation - https://www.odoo.com/forum/help-1/question/what-documentation-is-available-for-odoo-60312) but given the recent talk of crowdfunding the production of new themes:-
Over the next 3 months, we will also create three crowd funding
campaigns like the POS one. The campains will be on accounting, MRP and Themes for the CMS.
(Email from Fabien 30th June 2014)
it would appear that they are now not quite as ready to throw open the doors to invite designers to come in and build themes that can be charged for.
I think you should ask the question again to fabien in one of the community email discussion groups - the Odoo SA policy on themes needs clearly stating.
Kurt
For anyone that is interested I've put together some very basic instructions on how to get started with local theme development here: https://www.odoo.com/forum/help-1/question/new-bootstrap-theme-on-website-builder-58809