Google Chrome and Mozilla Firefox will soon support a new CSS media query called ‘prefers-color-scheme’ that lets’s sites know whether you prefer a dark mode, light mode, or have no preference. Apple has already added support for this feature in Safari 12.1.1. The feature could be used to fingerprint a user’s computer as it will reveal a setting in their operating system. Chrome developer Rune Lillesveen has asked security and privacy devs to provide approval before making the feature live.
Source: https://www.bleepingcomputer.com/news/software/chrome-will-allow-sites-to-check-your-os-dark-mode-settings/