Override 40x System error pages on Drupal 8

You have several mthodes to override 403 / 404 System pages.

Method 1. Set 403 / 404 pages.

You can set the 403 /404 page on "Basic site settings" page

Admin -> Configuration -> System -> Basic site settings

Path /admin/config/system/site-information

Method 2. Override system 40x Pages (/system/404 ...)

Step 1. Create a custom module (Here : 'mymodule').

Step 2. Create a basic page (Here : \Drupal\mymodule\Controller\MyErrorPage:on404).

Step 3. Add routing.yml file (Here : mymodule.routing.yml).

Example:

mymodule.system.404:

  path: '/system/404'

  defaults:

    _controller: '\Drupal\mymodule\Controller\MyErrorPage:on404'

    _title: 'Page not found - 404'

  requirements:

    _access: 'TRUE'