drupal 8 path to theme directory code example
Example: drupal 8 get theme path
$themeHandler = \Drupal::service('theme_handler');
$themePath = $themeHandler->getTheme($themeHandler->getDefault())->getPath();
$themeHandler = \Drupal::service('theme_handler');
$themePath = $themeHandler->getTheme($themeHandler->getDefault())->getPath();