Drupal8API.theme

<?php

function Drupal8API_preprocess_node(&$variables) { // $variables is an array where are stored all Drupal's variables
    $language = \Drupal::languageManager()->getCurrentLanguage()->getId(); // get the current language
    $variables['lang_code'] = $language;
    $variables['base_path'] = base_path(); // get the base path theme
    $variables['is_front'] = Drupal::service('path.matcher')->isFrontPage(); // store the front page url using the path matcher service
    $cuser = \Drupal::currentUser(); // get the current user;
    if(!$cuser->id()) {
        $variables['logged_in'] = "0"; // if false -> $variables['logged_in'] = "0"
    } else {
        $variables['logged_in'] = "1"; // if true -> $variables['logged_in] = "1"
    }
}

function Drupal8API_preprocess_html(&$variables) {
    $language = \Drupal::languageManager()->getCurrentLanguage()->getId(); // get the current language
    $node = \Drupal::routeMatch()->getParameter('node'); // fetch the fields of the current node 
    if($node) { // if it exists
        $variables['attributes']['class'][] = 'node-'.$node->id();// into $variables['attributes']['class'] I put the 'node-'.$node->id(); string
        $currentNodeA = $node->toArray(); // cast into Array
        $variables['attributes']['class'][] = 'node-type-'.$currentNodeA['type'][0]['target_id']; // into $variables['attributes']['class'] I put the 'node-type-'.$currentNodeA['type'][0]['target_id'] string
        $variables['attributes']['class'][] = 'i18n-'.$language; // into $variables['attributes']['class'] I put the 'i18n-'.$language; string
        if(isset($currentNodeA['field_structure'])) { // if is set the structure fields
            $variables['attributes']['class'][] = 'tax-'.$currentNodeA['field_structure'][0]['target_id']; // into $variables['attributes']['class'] I put the 'tax-'.$currentNodeA['field_structure'][0]['target_id']; string
        }
    }
}

?>