layout_custom_theme.module

<?php
/**
 * @file
 * Main file for Layout Custom Theme module.
 */

/**
 * Implements hook_config_info().
 */
function layout_custom_theme_config_info() {
  $prefixes['layout_custom_theme.settings'] = array(
    'label' => t('Layout Custom Theme settings'),
    'group' => t('Configuration'),
  );
  return $prefixes;
}

/**
 * Implements hook_form_FORM_ID_alter() for layout_settings_form.
 */
function layout_custom_theme_form_layout_settings_form_alter(&$form, &$form_state, $form_id) {
  // Get the layout object being modified.
  $layout = $form_state['layout'];

  // Get the config item if it exists.
  $config = config_get('layout_custom_theme.settings', $layout->name);

  // Define the form elements.
  $form['custom_theme'] = array(
    '#title' => t('Custom Theme'),
    '#type' => 'item',
    '#access' => !$layout->isDefault(),
  );
  $form['custom_theme']['enable'] = array(
    '#type' => 'checkbox',
    '#title' => t('Use custom theme on this layout.'),
    '#default_value' => (empty($config)) ? FALSE : TRUE,
    '#access' => !$layout->isDefault(),
  );

  // Get the options for the themes.
  $options = layout_custom_theme_get_themes();

  $form['custom_theme']['theme'] = array(
    '#type' => 'select',
    '#title' => 'Custom theme',
    '#options' => $options,
    '#empty_option' => '- Select theme -',
    '#default_value' => (empty($config)) ? '' : $config,
    '#states' => array(
      'visible' => array(':input[name="custom_theme[enable]"]' => array('checked' => TRUE)),
      'required' => array(':input[name="custom_theme[enable]"]' => array('checked' => TRUE)),
    ),
    '#access' => !$layout->isDefault(),
  );
  $form['actions']['submit']['#submit'][] = 'layout_custom_theme_form_layout_settings_form_alter_submit';
}

/**
 * Helper function to get custom theme options.
 *
 * @return array
 *   An array of all enabled, non-hidden themes with machine name and label.
 */
function layout_custom_theme_get_themes() {
  // Get current list of themes.
  $themes = system_rebuild_theme_data();
  // Define array to collect options.
  $custom_theme_options = array();
  // Loop through all themes.
  foreach ($themes as $theme) {
    // Check if enabled and not hidden.
    if ($theme->status && empty($theme->info['hidden'])) {
      // Add to array using machine name as key and label as value.
      $custom_theme_options[$theme->name] = $theme->info['name'];
    }
  }
  return $custom_theme_options;
}

/**
 * Save the settings.
 */
function layout_custom_theme_form_layout_settings_form_alter_submit($form, &$form_state) {
  // Get the layout object being modified.
  $layout = $form_state['layout'];

  // Define the config file.
  $config_name = 'layout_custom_theme.settings';

  // Check if custom theme is enabled on the form.
  if ($form_state['values']['custom_theme']['enable']) {
    // If so, set a config item for this layout with the value of this theme.
    config_set($config_name, $layout->name, $form_state['values']['custom_theme']['theme']);
  }
  else {
    // If not, delete the config item if it exists.
    config_clear($config_name, $layout->name);
  }
}

/**
 * Implements hook_custom_theme().
 */
function layout_custom_theme_custom_theme() {
  // Get the current item.
  $item = menu_get_item();

  // Check that an item has been returned and that user has access to it.
  if ($item && $item['access']) {
    // Get the layout in use by the current item.
    $layout = layout_get_layout_by_path(NULL, $item);
    // Attempt to retrieve a custom theme settings for this layout.
    $theme = config_get('layout_custom_theme.settings', $layout->name);
    // If a setting is returned, define this as the custom theme.
    if ($theme) {
      return $theme;
    }
  }
}