How do I change the font of the offer?

Overview

Offers use your theme's font. The app has no font setting.

• Product Add-Ons: add CSS in Appearance > Custom CSS with the .xb_checkbox_upsell_* classes. Click Show list to see them.
• Product Page Upsell: there is no Custom CSS field. Add CSS for #xb-product-upsell-root to your theme.

See the examples in How to customize the offer design.