Drupal themeing: overriding libraries that were already overridden

situation: building a subtheme of a subtheme, and need to override a library that was already overridden in the first subtheme.

there is a bug in library overrides, see https://www.drupal.org/project/drupal/issues/2642122

the answer is in reply #18 of that issue thread (thank you Jacine)
https://www.drupal.org/project/drupal/issues/2642122#comment-11201639
 

libraries-override:
contextual/drupal.contextual-links:
  css:
    component:
      /core/themes/stable/css/contextual/contextual.module.css: false
    theme:
      /core/themes/stable/css/contextual/contextual.theme.css: css/overrides/contextual.theme.css
      /core/themes/stable/css/contextual/contextual.icons.theme.css: false

And this is the logic:

1. Use the original module (or core) namespace for library name.
2. Use the path of the most recent override (in my case Stable) as the key
3. Use the full path to the file