Version 2.2 of the custom 404 error page plugin 404page introduced a native mode that can be used by theme developers to get full control over the plugin.
Activating the native mode causes the plugin to disable the Operating Method setting on the settings page and to not generate any output in case of a 404 error. The plugin provides PHP functions to manually display the title and the content of the seleted 404 error page. The native mode is useful if your theme does not make use of the WordPress Template System which means you don’t have a
404.php in your theme directory or you have but you don’t let WordPress itself decide which template file to use.
In that case the plugins Compatibility Mode may do trick – or not. It depends on your Theme if 404page is able to handle it. Supporting the 404page plugin natively would be the best solution. How to use the native mode is explained in the plugins documentation. If you have any problems with it, feel free to contact me.
Twenty Sixteen 404page Native Mode Demo
To demonstrate the native mode I’ve put together a Child Theme based on the Twenty Sixteen Theme. This does not really make sense because the Twenty Sixteen Theme works well with 404page. But it’s just for the purposes of demonstrating how native mode works.
You can download the Demo Theme from GitHub.
Suggestion for developers
If the Theme you’re developing is publicly available you can send me hint if you want me to mention your Theme.