You need to edit the CSS stylesheet (style.css). Its a very straightforward thing to do you’ll find it in the wp-content folder under your theme. remember to save a backup before you edit it. (style.css.bak)
I added the Jetpack plugin and got into the Custom CSS module, but when I entered in that code you added with your comment, nothing visually happened. I thought it might back the typeface color black, or white, or something?