If you are using jetpack, it has a CSS editor, but I’ve personally never used any, but perhaps this one might help?
http://wordpress.org/plugins/simple-custom-css/screenshots/
You may also want to consider using the child theme that comes with this theme…there’s a tutorial for it as well, plus you can read more about using child themes here:
http://codex.wordpress.org/Child_Themes
For actual font plugins that lets you choose fonts, sizes, and more, you may want to check out this one as it appears it’s a very popular one:
http://wordpress.org/plugins/wp-google-fonts/
