Show downloads only from 'themes' category. Taxonomy is called 'download_category'.

kultalusikka_front_page.php

<?php
/* Show downloads only from 'themes' category. Taxonomy is called 'download_category'. */
add_filter( 'kultalusikka_front_page_download_arguments', 'my_front_page_arguments' );

function my_front_page_arguments( $download_args ) {

  $download_args['tax_query'] = array(
  array(
    'taxonomy' => 'download_category',
    'field' => 'slug',
    'terms' => 'themes'
    )
  );

  return $download_args;

}
?>