This completely depends on the reader's lighting conditions and time of day.

If I'm sitting on my laptop in daylight, outdoors or in a room with large windows, I'd really like to max out the contrast while reading. Later in the day, the "Night Light" feature that's built into Windows 11 will reduce the contrast and increase the color temperature towards a yellower hue. (In past years I used f.lux for this.)

This kind of adjustment is not something each website should try to do. It's a system-wide setting that needs to take into account e.g. local sunrise/sundown times, like Night Light does. (And on iPhones, there's a light sensor that takes care of this.)