Cynically speaking: Desirable behaviour: hell no. Expected behaviour: whatever's least convenient for users, and maximally violates the principle of least astonishment. Less cynically: I don't understand dark mode. It's not something I felt a need for, before it came out, or voluntarily use now that it exists. (Sometimes it's on by default, or otherwise intrudes on me.) So I can't speak to what someone who valued dark mode would want.