Source

File: wp-includes/theme.php

function register_theme_directory( $directory ) {
	global $wp_theme_directories;

	if ( ! file_exists( $directory ) ) {
		// Try prepending as the theme directory could be relative to the content directory.
		$directory = WP_CONTENT_DIR . '/' . $directory;
		// If this directory does not exist, return and do not register.
		if ( ! file_exists( $directory ) ) {
			return false;
		}
	}

	if ( ! is_array( $wp_theme_directories ) ) {
		$wp_theme_directories = array();
	}

	$untrailed = untrailingslashit( $directory );
	if ( ! empty( $untrailed ) && ! in_array( $untrailed, $wp_theme_directories, true ) ) {
		$wp_theme_directories[] = $untrailed;
	}

	return true;
}