Use Fonts in Canvas Page Builder
Mainly the font settings in Canvas Page Builder are set using the Misc Font settings in Canvas Theme Settings.
There are 2 exceptions to this. From version 1 the Page Builder widget options under Canvas Settings > Canvas Extensions > Page Builder Widgets set the paragraph text (widget text) and Heading 3 (wigdet title).
As well as this you can use the Custom Typography shortcode in Canvas. More information on this can be found here.