There is official PWA plugin released and maintained by Google. Please use this WordPress plugin instead - https://wordpress.org/plugins/pwa/

WordPress Plugin to enable PWA features.

Once the plugin installed, go to Appearance > Customize > PWA WordPress to setup options.

Web app manifest example output:

{
  "background_color": "#ffffff",
  "description": "Just another WordPress site",
  "display": "standalone",
  "lang": "en-US",
  "name": "Site Name",
  "short_name": "WordPress",
  "start_url": "https://example.com",
  "theme_color": "#000000",
  "icons": [
    {
      "src": "https://example.com/wp-content/uploads/2018/09/cropped-icon-512-1-48x48.png",
      "sizes": "48x48"
    },
    {
      "src": "https://example.com/wp-content/uploads/2018/09/cropped-icon-512-1-192x192.png",
      "sizes": "192x192"
    },
    {
      "src": "https://example.com/wp-content/uploads/2018/09/cropped-icon-512-1-256x256.png",
      "sizes": "256x256"
    },
    {
      "src": "https://example.com/wp-content/uploads/2018/09/cropped-icon-512-1.png",
      "sizes": "512x512"
    }
  ]
}

From above entries, following fields can be manage from customizer options:

• name - Default will be get_bloginfo( 'name' )
• short_name - Default will be get_bloginfo( 'name' )
• description - Default will be get_bloginfo( 'description' )
• theme_color - Default will be #000000
• background_color - Default will be #ffffff

• pwa_ready_manifest To manage above manifest entries.
• pwa_wp_plugin_get_theme_color Control theme color.
• pwa_wp_plugin_get_background_color Control background color.

A rel="manifest" with link eg. https://example.com/?pwa_wp_plugin_manifest=1 will be added to wp_head.

Icon will be fetch from site icon. Go to Appearance > Customize > Site Identity > Site Icon to setup icons.

In Progress.