Inject a theme setting form into a block
You can create a custom block: https://www.drupal.org/docs/8/creating-custom-modules/creating-custom-blocks/create-a-custom-block

In the build() method, you can return the form:

public function build() {
  return \Drupal::formBuilder()-&gt;getForm('Drupal\system\Form\ThemeSettingsForm');
}