Drupal 8 config files

By gradually diffing as I make changes I want to discover which yml files I'd need to "play in" building a site piece by piece. Whist I could just build a site and then export/import the entire package, I wanted to gain better understanding so that I could, in future, know to deploy some pieces but not others, or to be able to make changes just to the yml files and rebuild the site entirely.

1. Selecting a new theme instead of the default

The following yml files become relevant:

Programatically adding content - Core Media images

In trying to build a fully repeatable Drupal 8 site build, as well as wanting to include config, modules and themes, I want to be to insert content. This will allow boiler-plate or test content to be inserted, or will allow me to quickly build out a new version of my target site.  

In the first instance I was looking for a way to insert a image into a Core Media Image node.