Doesn't modern CSS allow you to follow system settings?

making it a per-profile settings does not exclude making it available per-device or per-visit. it's not especially difficult to store a flag in localStorage or a cookie. hell, you can even use CSS to detect OS dark mode preference settings and have it change automatically with no user input at all.

I just want them to follow system default for guest user(s).

they probably require an account because they have a system set up in a way where it's mostly a server-side app and ui settings are only stored on the server

This makes more sense for signed in user(s).

No dark mode unless you register, is what I consider to be a asshole design choice.