Load entity term in xxx.theme
<?php
if (\Drupal::routeMatch()->getRouteName() == 'entity.taxonomy_term.canonical') {
// load the term entity and get the data from there
$term = \Drupal::routeMatch()->getParameter('taxonomy_term');
$variables['term_id'] = $term->id();
$variables['term_description'] = $term->getDescription();
}
?>