Source

File: wp-admin/includes/update.php

function get_theme_updates() {
	$current = get_site_transient( 'update_themes' );

	if ( ! isset( $current->response ) ) {
		return array();
	}

	$update_themes = array();
	foreach ( $current->response as $stylesheet => $data ) {
		$update_themes[ $stylesheet ]         = wp_get_theme( $stylesheet );
		$update_themes[ $stylesheet ]->update = $data;
	}

	return $update_themes;
}