Blimey, and I thought I made it dead easy :D
1) Export the theme element:
ACP -> Styles Category -> Themes -> Export (next to relevant theme) -> select relevant options, store or download, archive type, hit submit
Problem with above is that the user then has to create a new style (or update an existing style) using the new them they've imported
2) Export the style:
ACP -> Styles Category -> Styles -> Export (next to the relevant style using the relevant theme) -> Include only the theme, not the template or the imageset (both of which are subSilver and thus available to all users by default), select store or download, archive type, hit submit.
The resulting archive will contain only the theme files but will install as a new style using the exsting subSilver template set and imageset (or it'll complain if they are missing)