A search of the forum got me several discussions of how to adjust font size, including this one, which gives several options for mobile: Change font size on mobile - #6 by wbullock . Here’s another: Change font size
Whether those selectors will work on Digest, I’m not sure off the top of my head, but if you right click and inspect one of your pages, you can figure out what selectors will target the font you’re trying for.
Please try it out and post if you get stuck, being sure to show what you’ve already tried! 