shlink/module/Core/templates/error/404.phtml

<?php $this->layout('ShlinkCore::layout/default') ?>

<?php $this->start('title') ?>
  URL Not Found
<?php $this->end() ?>

<?php $this->start('stylesheets') ?>
  <style>
    p {margin-bottom: 20px;}
    body {text-align: center;}
  </style>
<?php $this->end() ?>

<?php $this->start('main') ?>
  <h1>404</h1>
  <hr>
  <h3>Page not found.</h3>
  <p>The page you requested could not be found.</p>
<?php $this->end() ?>