Loop para adicionar o nome da categoria na classe.

Classe exclusiva para categoria no Wordpress

<?php 
  $args = array(
    'post_type'      => 'post',
    'posts_per_page' => 100,
    'orderby'        => 'date',
    'order'          => 'DESC'
  );

  $loop = new WP_Query( $args );
  while ( $loop->have_posts() ):
    $loop->the_post();

  $postcat = get_the_category();

  if ( ! empty( $postcat ) ) {
      $category = $postcat[0]->slug ;   
      $category_name = $postcat[0]->name ;   
  }
?>

  <div class="<?= $category ?>">
  </div>
  
<?php endwhile; ?>
<?php wp_reset_query(); ?>