By defaults 9 basic fonts are always available : they are implemented on most device and their use will not affect the page loading time.
However to give your page a unique look and feel you may want to use different fonts.
Frog allow you to add any font from fonts.google.com
To add a font
Copy desired font URL of the font on Google Fonts
Paste the URL in the field and press add.
Font is now available in the theme font editor.
... and in the page builder of your funnel applied with that specific theme.
To keep a fast loading page : we strongly encourage you to limit your use of cutom fonts.