Font Stack
System fonts are loaded (Apple / Windows) and fallbacks are in place for older systems. Legacy themes use "Open Sans" as the default font.
Font
Notes
-apple-systemDefault Apple device font BlinkMacSystemFontFall back Apple device font "Segoe UI"Default Microsoft device font
HelveticaLegacy Apply Devices ArialLegacy Microsoft Devices sans-serifGeneric "catch all" "Apple Color Emoji"Support for Apple Emoji "Segoe UI Emoji"Support for Microsoft Emoji "Segoe UI Symbol"Support for Generic Emoji Custom Fonts
You can add custom fonts to your site via a webfont service such as https://fonts.google.com/or https://www.fonts.com/or via a custom implementation via using Scripts & Styles.
Refer to your style guide for further guidance as to usage on your site.