This "theme" has things used for both the main web page as well as the workbooks.
It consists of data and shortcode, but no page layouts - so it isn't really a theme, it's really a "theme mixin" (although, arguably, 559Theme isn't a complete theme either). Hugo has no terminology for a "partial theme".
Note: the workbooks.toml file (with the rubrics) has been moved to a separate repo.