Through the prefers-color-scheme CSS media query [1], a draft [2] that is already implemented in mainstream browsers.

Websites can write stylesheets specifically for light or dark preferences. Browsers currently derive this information from the operating system's preferences.

[1]: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/pref... [2]: https://drafts.csswg.org/mediaqueries-5/#prefers-color-schem...