Remove use of deprecated getBaseThemes()

Problem/Motivation

\Drupal\Core\Extension\ThemeExtensionList::getBaseThemes() is deprecated and will be removed in Drupal 12. See
https://www.drupal.org/node/3413187 →

We should remove usage of that method.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes