So I don't want to dive into the whys behind what I need to do but I have a very important piece of code that I need to insert into the 404 page regardless of the theme as a replacement of the content-area div. So below the get_header in replacement of the content-area and before the get_sidebar/get_footer.
Currently I have to install this piece of code into every themes 404 page before I can install it on my multisite, I'd like to accomplish this through a plugin but can't seem to override the themes 404 page.