You can save all the elements you are talking about in a Template. I don't think you are really talking about what Word calls a Theme. Templates, including Styles and background colors can be shared. Style Sets can be shared.

Look at Templates in Microsoft Word.

See http://www.wordbanter.com/showthread.php?t=142368

A theme is all of the things you discuss that can be applied to an individual document or template. You can, instead, apply your theme to your templates.

If you want your users to be able to change the look and feel of their own documents that they do not create from one of your templates to match your theme, you would be relying on skills far in excess of those needed to install a theme, IMO.