Pastel, light, dark and automatic

Automatic is the default and follows whatever the phone is set to: your light palette during the day, the dark theme at night. Pastel is an off-white background with soft lines, light is the same layout in cooler greys, and dark is dark whatever the phone says. Whatever you choose here is a starting point rather than a rule, because anyone can override it on their own device under Account, and a preference about screen brightness belongs to whoever is looking at the screen. Text on a brand-coloured button is picked from that colour's luminance rather than assumed, so a pastel brand gets dark text and stays readable in either theme. Status colours stay saturated in every scheme because they carry meaning: overdue has to look overdue.