The app shows its light theme on every device for now. A device set to dark mode still gets the light theme.
There is no theme setting
There is nothing to change. The Theme entry that used to sit in your account menu is hidden while the app has one theme.
Where you see it
The light theme is used on every page of the app, on the sign-in screens, and on the public pages such as pricing and the waitlist. Your browser's or device's dark setting does not change it.