UPDATE: a client confirmed this plugin works to change font https://wordpress.org/plugins/easy-google-fonts/
In order to add a font that does not come with the theme, you need to install the font first.
1 copy the font files in the theme root (where is style.css)
2. In style.css stylesheet add the new CSS declarations like in the section “Using The Font You Want” from this link: http://www.w3schools.com/css/css3_fonts.asp
3. Change the style css for the places where you want to use your new font
Alternatively, you can edit libs/custom_general_font.php and replace the PHP variable $general font with your font name.