This is the theme setting for your Error (404) Page.
Go to Appearance and click Customize. On customize click on ‘Error Page’
404 Error Text
You can customize your error text in this field
Upload 404 Image
You can upload image for your error page of your choice.
Enter the 404 error description here
Customize your error page description in this field
Show/Hide 404 Menu/Search
Choose whether to show or hide menu or search for the error page