[DX] Allow `theme_get_setting()` to return a default value - like `settings_get()` and `update_variable_get()` do

When trying to set up a settings form for a theme (using HOOK_form_system_theme_settings_alter() in a theme-settings.php file), I very often find myself doing something like this:

function MYTHEME_form_system_theme_settings_alter(&$form, &$form_state, $form_id = NULL) {
  $my_setting = theme_get_setting('my_setting');
  $form['my_setting'] = array(
    '#type' => 'checkbox',
    '#title' => t('Some setting'),
    '#default_value' => isset($my_setting) ? $my_setting : TRUE,
  );
}

I would like to be able to do the following instead:

function MYTHEME_form_system_theme_settings_alter(&$form, &$form_state, $form_id = NULL) {
  $form['my_setting'] = array(
    '#type' => 'checkbox',
    '#title' => t('Some setting'),
    '#default_value' => theme_get_setting('my_setting', 'MYTHEME', TRUE),
  );
}

In other words, change the function to include a 3rd, optional $default parameter so that it is theme_get_setting($setting_name, $theme = NULL, $default = NULL).

Similarly, I would like to be able to do that with config_get(), so change it to this: config_get($config_file, $option = NULL, $default = NULL).