Currently blocked on:
Problem/Motivation
Reason #2999721: [META] Deprecate the legacy include files before Drupal 9.
Proposed resolution
Deprecate theme_get_setting() and move its logic into ThemeHandlerInterface. Or a new service?
Remaining tasks
None.
User interface changes
None.
API changes
New methods in ThemeHandlerInterface:
::getThemeSetting()::clearThemeSettingCache()
Data model changes
None.
Release notes snippet
N/A