You can change the main font-family of the theme by clicking on Appearance -> Theme Options -> Advanced Settings.
To customize the font-size, please put your Custom CSS to Appearance -> Theme Options -> Advanced Settings -> Custom CSS.
Some suggestions for you can also get help from mirror font generator :

#wilcity-menu .menu-item{ font-size: 14px; } h1{ font-size: 14px; } h2{ font-size: 14px; } h3{ font-size: 14px; } h4{ font-size: 14px; } h5{ font-size: 14px; } h6{ font-size: 14px; }

You can also use Google Console to generate Custom Code yourself: