WordPress PWA compatibility plugin for the Lovecraft theme by Anders Norén. For use with the PWA feature plugin: https://wordpress.org/plugins/pwa/

offline.css

body.offline .toggles {
	display: none;
}

offline.php

<?php
/**
 * Offline template.
 *
 * @package Google\PWA_Lovecraft_Theme_Compat
 */

// Prevent showing nav menus.
add_filter( 'has_nav_menu', '__return_false' );
add_filter( 'wp_list_pages', '__return_empty_string' );

?>

<?php get_header(); ?>

<div class="wrapper section">

	<div class="section-inner">

		<div class="content">

			<div class="post single">

				<div class="post-inner">

					<div class="post-header">

						<h1 class="post-title"><?php _e( 'Offline', 'pwa' ); ?></h1>

					</div><!-- .post-header -->

					<div class="post-content">

						<?php wp_service_worker_error_message_placeholder(); ?>

					</div>

				</div><!-- .post-inner -->

			</div><!-- .post -->

		</div><!-- .content -->

		<?php //get_sidebar(); ?>

		<div class="clear"></div>

	</div><!-- .section-inner -->

</div><!-- .wrapper.section -->

<?php get_footer(); ?>

pwa-lovecraft-theme-compat.php

<?php
/**
 * PWA Lovecraft Theme Compat plugin bootstrap.
 *
 * @package   Google\PWA_Lovecraft_Theme_Compat
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2020 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: PWA Lovecraft Theme Compat
 * Plugin URI:  https://gist.github.com/westonruter/26667d26ea34ef8e9f5e1053a747c73f
 * Description: Plugin to add <a href="https://wordpress.org/plugins/pwa/">PWA plugin</a> compatibility to the <a href="https://wordpress.org/themes/lovecraft/">Lovecraft</a> theme by Anders Norén.
 * Version:     0.1
 * Author:      Weston Ruter, Google
 * Author URI:  https://weston.ruter.net/
 * License:     GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 */

namespace Google\PWA_Lovecraft_Theme_Compat;

add_filter( 'after_setup_theme', __NAMESPACE__ . '\add_hooks', 20 );

/**
 * Add hooks.
 */
function add_hooks() {
	if ( 'lovecraft' !== get_template() ) {
		return;
	}
	add_filter( 'template_include', __NAMESPACE__ . '\filter_template_include_to_override_offline_template', 20 );
	add_action( 'wp_enqueue_scripts', __NAMESPACE__ . '\add_offline_template_style', 11 );
	add_filter( 'wp_offline_error_precache_entry', __NAMESPACE__ . '\filter_offline_error_precache_entry' );
}

/**
 * Locate offline template.
 *
 * @return string
 */
function locate_offline_template_path() {
	return __DIR__ . '/offline.php';
}

/**
 * Filter the template include file to override the offline template.
 *
 * @param string $include Template include file.
 * @return string Filtered template include file.
 */
function filter_template_include_to_override_offline_template( $include ) {
	if ( function_exists( 'is_offline' ) && \is_offline() ) {
		$include = __DIR__ . '/offline.php';
	}
	return $include;
}


/**
 * Add offline template styles.
 */
function add_offline_template_style() {
	if ( function_exists( 'is_offline' ) && \is_offline() ) {
		wp_add_inline_style( 'lovecraft_style', file_get_contents( __DIR__ . '/offline.css' ) );
	}
}

/**
 * Filter the offline error precache entry to ensure revision is bumped when changes are made to the template.
 *
 * @param array $entry Entry.
 * @return array Entry.
 */
function filter_offline_error_precache_entry( $entry ) {
	$entry['revision'] .= ';' . __NAMESPACE__ . '-' . filemtime( locate_offline_template_path() );
	return $entry;
}

Installation instructions: https://gist.github.com/westonruter/6110fbc4bef0c4b8c021a112012f7e9c

For a AMP compatibility, see https://gist.github.com/westonruter/08bea805f2a31e7934dfeeb4e36bb662