Can a theme leverage block config Yaml files when installed for the first time?
It turns out tim-plunkett was correct in his comment above. I used the config export function at /config/development/configuration/full/export to export all the block settings for my theme. 

I then took all the generated YAML files and placed them in a new folder within my theme at /config/install. I then uninstalled the theme and then reinstalled it as the default. Voila, all my blocks, i.e., menu, help etc... were in their designated regions according to what had been exported in the YAML files. 

Some examples of the file names are:

block.block.gratis_help.yml
block.block.gratis_main_menu.yml
block.block.gratis_branding.yml