Reader is one of those features in mobile Safari I rarely use, but greatly appreciate it when I do. When activated, iOS strips away any ads and unnecessary clutter from the article, making it easier to read.
With iOS 9, Apple has added new options to Safari's Reader mode. The new settings include the ability to pick your preferred font and background color.
To change how articles are displayed in Reader mode, open Safari and tap on the Reader button. Next tap on the aA icon in the far-right corner of the address bar.
A submenu will show up giving you a list of the various background colors and font options. Notice you can also adjust font size by tapping on the two buttons sitting above the color options.
Below you'll find an example of all three colors now available in Reader mode: