This is how you can get in a custom or in a preprocess function the absolute path of an icon that you want to use in your custom theme.
global $base_url; $default_theme_name = \Drupal::config('system.theme')->get('default'); $theme_path = \Drupal::service('extension.list.theme')->getPath($default_theme_name); $popular_icon = $base_url . '/' . $theme_path . '/assets/css/images/popular_icon.svg';
The "assets/css/images" path is where i have the custom icons or images that i want to use for my theme.
The $base_url can be skipped if you just want to use the relative path.