function wp_update_themes( $extra_stats = array() ) {
	if ( wp_installing() ) {
		return;
	}

	// Include an unmodified $wp_version.
	require ABSPATH . WPINC . '/version.php';

	$installed_themes = wp_get_themes();
	$translations     = wp_get_installed_translations( 'themes' );

	$last_update = get_site_transient( 'update_themes' );

	if ( ! is_object( $last_update ) ) {
		$last_update = new stdClass;
	}

	$themes  = array();
	$checked = array();
	$request = array();

	// Put slug of current theme into request.
	$request['active'] = get_option( 'stylesheet' );

	foreach ( $installed_themes as $theme ) {
		$checked[ $theme->get_stylesheet() ] = $theme->get( 'Version' );

		$themes[ $theme->get_stylesheet() ] = array(
			'Name'       => $theme->get( 'Name' ),
			'Title'      => $theme->get( 'Name' ),
			'Version'    => $theme->get( 'Version' ),
			'Author'     => $theme->get( 'Author' ),
			'Author URI' => $theme->get( 'AuthorURI' ),
			'Template'   => $theme->get_template(),
			'Stylesheet' => $theme->get_stylesheet(),
		);
	}

	$doing_cron = wp_doing_cron();

	// Check for update on a different schedule, depending on the page.
	switch ( current_filter() ) {
		case 'upgrader_process_complete':
			$timeout = 0;
			break;
		case 'load-update-core.php':
			$timeout = MINUTE_IN_SECONDS;
			break;
		case 'load-themes.php':
		case 'load-update.php':
			$timeout = HOUR_IN_SECONDS;
			break;
		default:
			if ( $doing_cron ) {
				$timeout = 2 * HOUR_IN_SECONDS;
			} else {
				$timeout = 12 * HOUR_IN_SECONDS;
			}
	}

	$time_not_changed = isset( $last_update->last_checked ) && $timeout > ( time() - $last_update->last_checked );

	if ( $time_not_changed && ! $extra_stats ) {
		$theme_changed = false;

		foreach ( $checked as $slug => $v ) {
			if ( ! isset( $last_update->checked[ $slug ] ) || (string) $last_update->checked[ $slug ] !== (string) $v ) {
				$theme_changed = true;
			}
		}

		if ( isset( $last_update->response ) && is_array( $last_update->response ) ) {
			foreach ( $last_update->response as $slug => $update_details ) {
				if ( ! isset( $checked[ $slug ] ) ) {
					$theme_changed = true;
					break;
				}
			}
		}

		// Bail if we've checked recently and if nothing has changed.
		if ( ! $theme_changed ) {
			return;
		}
	}

	// Update last_checked for current to prevent multiple blocking requests if request hangs.
	$last_update->last_checked = time();
	set_site_transient( 'update_themes', $last_update );

	$request['themes'] = $themes;

	$locales = array_values( get_available_languages() );

	/**
	 * Filters the locales requested for theme translations.
	 *
	 * @since 3.7.0
	 * @since 4.5.0 The default value of the `$locales` parameter changed to include all locales.
	 *
	 * @param array $locales Theme locales. Default is all available locales of the site.
	 */
	$locales = apply_filters( 'themes_update_check_locales', $locales );
	$locales = array_unique( $locales );

	if ( $doing_cron ) {
		$timeout = 30;
	} else {
		// Three seconds, plus one extra second for every 10 themes.
		$timeout = 3 + (int) ( count( $themes ) / 10 );
	}

	$options = array(
		'timeout'    => $timeout,
		'body'       => array(
			'themes'       => wp_json_encode( $request ),
			'translations' => wp_json_encode( $translations ),
			'locale'       => wp_json_encode( $locales ),
		),
		'user-agent' => 'WordPress/' . $wp_version . '; ' . home_url( '/' ),
	);

	if ( $extra_stats ) {
		$options['body']['update_stats'] = wp_json_encode( $extra_stats );
	}

	$url      = 'http://api.wordpress.org/themes/update-check/1.1/';
	$http_url = $url;
	$ssl      = wp_http_supports( array( 'ssl' ) );

	if ( $ssl ) {
		$url = set_url_scheme( $url, 'https' );
	}

	$raw_response = wp_remote_post( $url, $options );

	if ( $ssl && is_wp_error( $raw_response ) ) {
		trigger_error(
			sprintf(
				/* translators: %s: Support forums URL. */
				__( 'An unexpected error occurred. Something may be wrong with WordPress.org or this server&#8217;s configuration. If you continue to have problems, please try the <a href="%s">support forums</a>.' ),
				__( 'https://wordpress.org/support/forums/' )
			) . ' ' . __( '(WordPress could not establish a secure connection to WordPress.org. Please contact your server administrator.)' ),
			headers_sent() || WP_DEBUG ? E_USER_WARNING : E_USER_NOTICE
		);
		$raw_response = wp_remote_post( $http_url, $options );
	}

	if ( is_wp_error( $raw_response ) || 200 !== wp_remote_retrieve_response_code( $raw_response ) ) {
		return;
	}

	$new_update               = new stdClass;
	$new_update->last_checked = time();
	$new_update->checked      = $checked;

	$response = json_decode( wp_remote_retrieve_body( $raw_response ), true );

	if ( is_array( $response ) ) {
		$new_update->response     = $response['themes'];
		$new_update->no_update    = $response['no_update'];
		$new_update->translations = $response['translations'];
	}

	set_site_transient( 'update_themes', $new_update );
}