Sub-theme does not inherit base theme's overridden libraries

Problem/Motivation

Given you have these themes:

• base_theme
• sub_theme

A sub-theme does not correctly inherit its base theme's libraries-override defined in base_theme.info.yml. If the default theme is base_theme, it works, but if you set sub_theme to default, the original asset will be rendered.

The only possible workaround I found, is to redefine (i.e. copy and paste) the overrides in the subtheme as well, but as the base assets and overrides can change later, this will cause your subtheme definitions to break.

I tested with overriding system/base assets, as base_theme by default inherits Stable. Tried to test with Bartik set to base theme in base_theme.info.yml, but it did not even work when base_theme was set to default theme. (This might be another issue)

The documentation does not mention this: https://www.drupal.org/docs/8/theming-drupal-8/creating-a-drupal-8-sub-t...

Here's my code:

base_theme.info.yml:

type: theme
name: Base theme
core: 8.x

libraries-override:
  system/base:
    css:
      component:
        /core/themes/stable/css/system/components/ajax-progress.module.css: css/ajax-progress.overridden.css

sub_theme.info.yml:

type: theme
name: Sub theme
core: 8.x
base theme: base_theme

Proposed resolution

The default theme should inherit everything defined in all of its parent themes, by common sense. This includes libraries-override too. Imagine having multiple projects (A, B, C) using the same base theme ("theme_root") which overrides a lot of system libraries, and you need only some minor theme tweaking in each project ("theme_a", "theme_b", etc...)

By the way, libraries-extend seems to work as expected.

Remaining tasks

• Further testing with base theme: false, Stark, Classy, Bartik.
• Fix inheriting library overrides.