Why isn't my sub-theme using its CSS file?
In Drupal 8 you setup your subtheme libraries in subtheme.libraries.yml. These libraries are then loaded from subtheme.info.yml.

You should check the paths in those files if they are correct. 

It's good practice to search for all occurrences of the base_theme's machine_name inside your subtheme folder and replace those with the machine_name of your subtheme.