I think this makes sense seeing as how login.htm and index.htm are in the themes folder, the error pages should be here too.
Its usually the UXer/front-enders job to make this look a bit nicer, and would be easy when distributing the theme package around.
or maybe make the default 404 look slightly more clean/netural/professional,
it’s a bit weird working with finance companies and when the app is down there are these weird little offbrand monsters in your screen.
I agree the default 404 could be more professional. However, I'd rather don't have the 404 just in my theme folder, as it then also becomes available at myapp.nl/404.html. I'm fine with the error_pages folder, but it would be nice those files exist by default on creation of a new project.