Like the title says, this add-on changes XenForo's default 404 error page to a slim good-looking one. You can change the error page as your liking just by editing the 'gfn404_error' template.
I've also added a back button and a 'Return Home' button for added functionality.
- Download & extract archive.
- Upload everything from the 'upload' directory.
- Import the add-on XML file from admin control panel.
Install the add-on with Chris D's or Waindigo's add-on installer