Starting yesterday, Twitter automatically switched to a dark or light theme based on your operating system settings. The feature appears to be only working Chromium-based browsers such as Google Chrome, Microsoft, Edge, and Brave. To control this feature, Twitter uses the ‘prefers-color-scheme’ CSS media query that allows web sites to modify a page’s theme and layout based on the operating system’s preferences. This new feature is a little buggy, as it breaks if you go into your Twitter settings and change the display preferences.
Source: https://www.bleepingcomputer.com/news/technology/twitter-now-autoswitches-to-dark-mode-based-on-your-os-settings/