function.phpに以下を追加

//記事の並び順
function my_orderby_post_date( $query ) {
  if( $query->is_main_query() ) {
    if( $query->is_home() || $query->is_category() ) {
      $query->set( 'order', 'desc' );
      $query->set( 'orderby', 'post_date' );
    }
  }
}
add_action( 'pre_get_posts', 'my_orderby_post_date' );