Issue #2845407 by xumepadismal, imclean, donaldp, Christopher Riley, noopal,...

    // Retrieve a diff of settings that override the defaults.

    $diff = DiffArray::diffAssocRecursive($data, $this->defaults);

    // Ensure core features are always present in the diff. The theme settings

    // form will not work properly otherwise.

    // Ensure core features and complex (array) settings are always present in

    // the diff. The theme settings form will not work properly otherwise.

    // @todo Just rebuild the features section of the form?

    foreach (['favicon', 'features', 'logo'] as $key) {

    $core_settings = ['favicon', 'features', 'logo'];

    $complex_settings = array_keys(array_filter($diff, 'is_array'));

    $all_complex_settings = array_unique(array_merge($core_settings, $complex_settings));

    foreach ($all_complex_settings as $key) {

      $arrays = [];

      $arrays[] = isset($this->defaults[$key]) ? $this->defaults[$key] : [];

      $arrays[] = isset($data[$key]) ? $data[$key] : [];