[zamadatix]

It's beta, the average user isn't supposed to see it yet:

"We want to share it with our community, especially our power users, and gather feedback so we can improve, iterate, and expand Dark Mode in the future."

One of the beta is "System setting" which pulls the default from the browsers "prefers-color-scheme" value (which by default pulls it's value from the system scheme). When it's ready to roll out fully this seems to be the likely path they are planning.