Modifying Upfront Child Themes

Is there a way to change the defaults content/images/stuff of one of these themes?

My in-development site network is for church related websites. I don't want to offer 4 or 5 theme options that have NOTHING to do with that. I just want to change all the boilerplate stuff so that when someone activates their new site and theme, it is more relevant to what the whole point of my MS network is.

I would be fine if I had to do this directly in the code. I'm happy to fork and hack away. But I can't find where any of this stuff is actually specified.