Tap AA again, and select one of the two dark mode color options. Tap AA once more to close. To turn off Reader for a particular website, tap the AA button in the address bar and select Hide Reader.

Dark mode won't change the color of the pages you visit (that's set by the site's developer), but it'll make the toolbar and settings pages much easier to look at, especially in low-light situation.

Dark Mode: iCloud.com will automatically match your device settings with a Light Mode or Dark Mode color scheme. Customize Your Background: Choose between different colors on the home page.