You can export/import site settings already:
It might take a bit of work to document each one, but a simple way might be to post a new topic here in Meta (suitably tagged etc) outlining all of the customisations using a structure like this:
- Design concept & demonstration site (if available)
- Theme as an exported file
- Theme Components as a list with a brief overview of changed settings
- Plugins with brief notes on the configuration of each
- Settings that have been changed from default as a .yml file