ThemeSettingsForm::submitForm

public ThemeSettingsForm::submitForm(array &$form, FormStateInterface $form_state)

public function submitForm(array &$form, FormStateInterface $form_state) {
  parent::submitForm($form, $form_state);

  $config_key = $form_state->getValue('config_key');
  $this->editableConfig = [$config_key];
  $config = $this->config($config_key);

  // Exclude unnecessary elements before saving.
  $form_state->cleanValues();
  $form_state->unsetValue('var');
  $form_state->unsetValue('config_key');

  $values = $form_state->getValues();

  // If the user uploaded a new logo or favicon, save it to a permanent location
  // and use it in place of the default theme-provided file.
  if (!empty($values['logo_upload'])) {
    $filename = file_unmanaged_copy($values['logo_upload']->getFileUri());
    $values['default_logo'] = 0;
    $values['logo_path'] = $filename;
  }
  if (!empty($values['favicon_upload'])) {
    $filename = file_unmanaged_copy($values['favicon_upload']->getFileUri());
    $values['default_favicon'] = 0;
    $values['favicon_path'] = $filename;
    $values['toggle_favicon'] = 1;
  }
  unset($values['logo_upload']);
  unset($values['favicon_upload']);

  // If the user entered a path relative to the system files directory for
  // a logo or favicon, store a public:// URI so the theme system can handle it.
  if (!empty($values['logo_path'])) {
    $values['logo_path'] = $this->validatePath($values['logo_path']);
  }
  if (!empty($values['favicon_path'])) {
    $values['favicon_path'] = $this->validatePath($values['favicon_path']);
  }

  if (empty($values['default_favicon']) && !empty($values['favicon_path'])) {
    $values['favicon_mimetype'] = $this->mimeTypeGuesser->guess($values['favicon_path']);
  }

  theme_settings_convert_to_config($values, $config)->save();
}