Currently there are no options to fully customize the 404 page withing the theme functionality.
We’ve added your request to our Wish list and will certainly add such opportunity in one of our future theme updates.
Thank you for the suggestion.
The 404 page is a default archive WordPress page and it has no page editor.
You can edit it’s Header style and layout type in the Theme options > General options > Layouts settings.
It’s Stunning header background image inherits from the Theme options > Header Options > Stunning header options.