Custom fonts that are available in your app bundle can be used in the Iterate survey view by passing their names to the Iterate.shared.configure method, like this:
Iterate.shared.configure(apiKey: YOUR_API_KEY, surveyTextFontName: "Merriweather-Regular", buttonFontName: "WorkSans-Regular")
The font specified in the buttonFontName parameter will be used in all survey interface buttons (question responses, previous / next buttons, etc). The font specified in surveyTextFontName will be used for all other survey text (question prompts, explanatory copy, etc).
True Type fonts and Open Type Fonts are supported. To find the correct font name to use, see Apple's documentation on custom fonts.