Probably because you're commenting without being familiar with how your browser sending information to pages to use a dark scheme for you. A website that supports "@media (prefers-color-scheme: dark)" is not really a maximalist design these days.
It's not dark mode by default. The text color is initially undefined and the background-color transparent. If the user's device is set to prefer dark mode, color and background color are set to be white text on a dark background.
It would be good if it was possible for a reader to choose to use the opposite of their device setting but that would would less minimal (needs a UI, JavaScript, and client-side storage of the setting).