A theme generated with starterkit is based on stable9 and creates templates and libraries with some enhanced defaults, similar to the Classy theme.
Themes can override or extend other libraries.
When a sub theme tries to override a library that is already overridden by base theme, the base theme's file path must be used.
Instead of (css file defined by views_ui/admin.styling library):
library-override:
views_ui/admin.styling:
css:
theme:
css/views_ui.admin.theme.css
Use (css file defined by stable9 override):
library-override:
views_ui/admin.styling:
css:
theme:
/core/themes/stable9/css/views_ui/views_ui.admin.theme.css
It has to have the full file path from the docroot to work.