Safari’s Reader mode is a handy feature that makes it easier to focus on the contents of an article by stripping away everything but the body text and images. Up until now, its appearance options have been limited to a single font and a single background color. In El Capitan, though, you have options.
Go to an article page (the one you’re looking at now works!) and press the Reader button—it’s the one with four horizontal lines located on the left side of the address bar. Once in Reader mode, look for the display options button in the address bar, denoted by two A’s. Click it, and you can pick between several different fonts and color schemes.
Once you find the look that works for you, click anywhere else in your Safari window, then read away!