Why is the subtheme_style.css not being called?
Turns out, I had everything set up correctly, but hadn't toggled "ON" the style sheet. 


Navigate to: Site settings >> Appearance >> Settings >> Toggle Styles
Click "ON" checkbox next to New Theme global style. (all) - new_theme_style.css


I slept on it and woke early in the morning, remembering that not only do you need to enable your theme, you need to turn on the style sheet.