iOS does support that kind of extension. There's an extremely good app for creating/editing the custom CSS/JS https://github.com/quoid/userscripts and it can even sync with your other iDevices/Macs over iCloud.
EDIT: Maybe Userscripts' UI doesn't have an integrated text-editor when running on iPhone. It definitely does when running on Mac, and possibly on iPad.
iPhone browser extensions can only run in the Safari app, not inside other apps that embed it. I read HN through Feedly (which displays each article inside the app), and there’s no way to do any user customization.
EDIT: Maybe Userscripts' UI doesn't have an integrated text-editor when running on iPhone. It definitely does when running on Mac, and possibly on iPad.