Light, dark or auto — and why it's per device
Appearance is chosen on each device, not on your account, so your phone can follow the time of day while your desk stays light.
Open the menu on your avatar and choose Auto, Light or Dark. Auto follows the device.
Unlike language, appearance is remembered on the device, not on your account. A dark phone at night and a light screen at work is the usual wish, and the device knows its own setting best.