Change record status: 
Project: 
Introduced in branch: 
9.1.x
Introduced in version: 
9.1.0
Description: 

The \Drupal\Core\Config\ConfigImporter constructor argument $theme_handler replaced with $theme_installer of \Drupal\Core\Extension\ThemeInstallerInterface

All inherited classes should replace usage passing theme_installer service instead of theme_handler

Impacts: 
Module developers