I see that the template directive is not exhibited in the sample sabledocs.toml. I'm guessing that's because you haven't finalized how that would work.
I needed to use some different template files, and I ended up making this minor change.
The default template directory _default/ now appears fewer times in the code, and if template is specified in the config file, it is no longer required to be a subdirectory of the current working directory (but it still could be).
Details from here: https://github.com/markvincze/sabledocs/pull/20