themes-wordpress/libretto/404.php

<?php
/**
 * The template for displaying 404 pages (not found).
 *
 * @package Libretto
 */

get_header(); ?>

  <div id="primary" class="content-area">
    <div id="content" class="site-content" role="main">

      <section class="error-404 not-found">

        <div class="page-content">
          <p><?php esc_html_e( 'It looks like nothing was found at this location. Maybe try one of the links below or a search?', 'libretto' ); ?></p>

			<?php get_search_form(); ?>

			<?php the_widget( 'WP_Widget_Recent_Posts' ); ?>

			<?php if ( libretto_categorized_blog() ) : // Only show the widget if site has multiple categories. ?>
          <div class="widget widget_categories">
            <h2 class="widget-title"><?php esc_html_e( 'Most Used Categories', 'libretto' ); ?></h2>
            <ul>
            <?php
			  wp_list_categories( array(
				  'orderby'    => 'count',
				  'order'      => 'DESC',
				  'show_count' => 1,
				  'title_li'   => '',
				  'number'     => 10,
			  ) );
			?>
            </ul>
          </div><!-- .widget -->
			<?php endif; ?>

			<?php
			/* translators: %1$s: smiley */
			$archive_content = '<p>' . sprintf( esc_html__( 'Try looking in the monthly archives. %1$s', 'libretto' ), convert_smilies( ':)' ) ) . '</p>';
			the_widget( 'WP_Widget_Archives', 'dropdown=1', "after_title=</h2>$archive_content" );
			?>

			<?php the_widget( 'WP_Widget_Tag_Cloud' ); ?>

        </div><!-- .page-content -->
      </section><!-- .error-404 -->

    </main><!-- #main -->
  </div><!-- #primary -->

<?php get_footer(); ?>