Page.php

<?php
/**
 * Adds the ability to select a theme to use to render a page, and have it
 * cascade down the tree.
 */
class Page extends SiteTree {

	public static $db = array(
		'Theme' => 'Varchar'
	);

	public function getCMSFields() {
		$fields = parent::getCMSFields();

		$theme = new DropdownField(
			'Theme',
			null,
			SiteConfig::current_site_config()->getAvailableThemes(),
			null, null,
			'(Inherit from parent)');
		$fields->addFieldToTab('Root.Behaviour', $theme, 'ParentType');

		return $fields;
	}

}

class Page_Controller extends ContentController {

	public function init() {
		parent::init();

		$current = $this;
		while ($current && !$current->Theme) {
			$current = $current->ParentID ? $current->Parent() : false;
		}

		if ($current) {
			SSViewer::set_theme($current->Theme);
		}
	}

}