Source: includes/theme-integrations/theme-integration-loader.php

<?php

/**
 * Class Sensei_Theme_Integration_Loader
 *
 * Responsible for loading the corrent supported theme if a
 * support theme is installed.
 *
 * @package Views
 * @subpackage Theme-Integration
 * @author Automattic
 *
 * @since 1.9.0
 */
class Sensei_Theme_Integration_Loader {

	/**
	 * @var array
	 * Holding a reference core supported themes
	 */
	protected $themes;

	/**
	 * @var string
	 * reference to the theme currently active on this site
	 */
	protected $active_theme;

	public function __construct() {

		$this->setup_themes();
		$this->setup_currently_active_theme();

		add_action( 'init', array( $this, 'possibly_load_supported_theme_wrappers' ) );

	}

	/**
	 * Setup the theme slugs supported by Sensei Core
	 *
	 * @since 1.9.0
	 */
	private function setup_themes() {

		$this->themes = array(
			'twentyeleven',
			'twentytwelve',
			'twentythirteen',
			'twentyfourteen',
			'twentyfifteen',
			'twentysixteen',
			'twentyseventeen',
			'storefront',
		);

	}

	/**
	 * Get names of all themes supported by default.
	 *
	 * @since 1.12.0
	 */
	public function get_supported_themes() {
		return $this->themes;
	}

	/**
	 * Setup the currently active theme
	 *
	 * @since 1.9.0
	 */
	private function setup_currently_active_theme() {

		$this->active_theme = get_option( 'template' );

	}

	/**
	 * Remove default Sensei wrappers and load
	 * supported wrappers if the current theme is
	 * a theme we have an integration for within core.
	 *
	 * @since 1.9.0
	 */
	public function possibly_load_supported_theme_wrappers() {

		/**
		 * Allow developer to stop the loading of the default supported theme wrappers.
		 * After removing this you can follow the documentation on how to add theme support.
		 *
		 * @since 1.9.4
		 *
		 * @hook sensei_load_default_supported_theme_wrappers
		 *
		 * @param {bool} $load_default_supported_theme_wrappers Whether to load the default supported theme wrappers.
		 * @return {bool} Filtered value.
		 */
		$load_default_supported_theme_wrappers = apply_filters( 'sensei_load_default_supported_theme_wrappers', true );

		if ( in_array( $this->active_theme, $this->themes ) && $load_default_supported_theme_wrappers ) {

			// setup file and class names
			$supported_theme_class_file = trailingslashit( Sensei()->plugin_path ) . 'includes/theme-integrations/' . $this->active_theme . '.php';
			$supported_theme_class_name = 'Sensei_' . ucfirst( $this->active_theme );

			// load the file or exit if there is no file for this theme
			if ( ! file_exists( $supported_theme_class_file ) ) {
				return;
			}
			include_once $supported_theme_class_file;
			include_once dirname( __FILE__ ) . '/twentytwelve.php';
			// initialize the class or exit if there is no class for this theme
			if ( ! class_exists( $supported_theme_class_name ) ) {
				return;
			}
			$supported_theme = new $supported_theme_class_name();

			// remove default wrappers
			remove_action( 'sensei_before_main_content', array( Sensei()->frontend, 'sensei_output_content_wrapper' ), 10 );
			remove_action( 'sensei_after_main_content', array( Sensei()->frontend, 'sensei_output_content_wrapper_end' ), 10 );

			// load the supported theme wrappers
			add_action( 'sensei_before_main_content', array( $supported_theme, 'wrapper_start' ), 10 );
			add_action( 'sensei_after_main_content', array( $supported_theme, 'wrapper_end' ), 10 );
		}
	}

}