First, go into you template’s General tab and turn off Optimize CSS and JS. Save changes. Then, with Inspect in Chrome browser (similar in FF etc), you can test settings for font family, point size, line-height etc. You do this by selecting some text, then right click with your mouse and select Inspect.
In the lower part of your screen you’ll see the CSS applied to your article text. I noticed that you have different fonts set in different parts of your site. I would stick with one text font. Verdana is very legible at smallish sizes, for example. You then copy the change in settings you decide upon to your custom.css file to get a global change.
Good luck. Ask if you need more help.
Tim
P.S. More here: https://developers.google.com/web/tools/chrome-devtools/iterate/inspect-styles/?hl=en