Both settings live under Settings → Appearance, and both are per browser rather than per account — set them once on each device you use.
Light, dark or automatic
Auto follows your system appearance and is the default, so Fidaro turns dark when your phone or laptop does. Light and Dark pin it regardless.
Notify me when a response is ready
Long answers are worth walking away from. Turn this on and Fidaro raises a browser notification when a reply finishes while you are looking at something else — another tab, another app, a minimised window. If you are watching the chat, nothing fires; you can already see it.
Your browser will ask permission the first time. If you refuse, or later block notifications in browser settings, the row says so and the toggle turns itself off rather than pretending to be on.
The notification says nothing about your chat
Every one reads the same: A response is ready. No thread title, no snippet, no sender.
That is deliberate. Your operating system keeps notifications in a tray and a history, outside Fidaro and outside the encryption that protects your conversations. Anything put in a notification would sit there in plain text on a lock screen. So nothing goes in one.
There is also no push service involved. This is a plain browser notification raised by the page on your own device — no server sends it, and no third party is told that your reply finished. Several finishing at once collapse into a single entry, and clicking it brings you back to Fidaro.
On an iPhone or iPad
Safari only allows notifications for sites you have added to your Home Screen. Add Fidaro first, then turn the setting on inside the installed app — see Is there a Fidaro app?
Some browsers do not support notifications at all, in which case the row tells you rather than offering a toggle that could never work.