get_raw_theme_root( string $stylesheet_or_template, bool $skip_cache = false )

function get_raw_theme_root( $stylesheet_or_template, $skip_cache = false ) {
	global $wp_theme_directories;

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

	$theme_root = false;

	// If requesting the root for the current theme, consult options to avoid calling get_theme_roots()
	if ( ! $skip_cache ) {
		if ( get_option('stylesheet') == $stylesheet_or_template )
			$theme_root = get_option('stylesheet_root');
		elseif ( get_option('template') == $stylesheet_or_template )
			$theme_root = get_option('template_root');
	}

	if ( empty($theme_root) ) {
		$theme_roots = get_theme_roots();
		if ( !empty($theme_roots[$stylesheet_or_template]) )
			$theme_root = $theme_roots[$stylesheet_or_template];
	}

	return $theme_root;
}