[deleted] which error pages are you referring to? 404/403 page with "Go back to homepage"?
Unfortunately there's no user-friendly way of changing any of the error pages at this time.
Using the Flarum PHP extension API, you could register an alias for the view file used by the general 404/403 page and include different inline CSS in it.
There are other 500 error pages that can theoretically be customized, but depending what kind of error happened the custom code might not get called.
