Source

File: wp-includes/theme.php

function get_theme_roots() {
	global $wp_theme_directories;

	if ( ! is_array( $wp_theme_directories ) || count( $wp_theme_directories ) <= 1 ) {
		return '/themes';
	}

	$theme_roots = get_site_transient( 'theme_roots' );
	if ( false === $theme_roots ) {
		search_theme_directories( true ); // Regenerate the transient.
		$theme_roots = get_site_transient( 'theme_roots' );
	}
	return $theme_roots;
}