You can change your app's default font in your device's Settings. (Display>Font style > and a List of available fonts appears)
The device fault font will be used for Labels and TextBoxes etc ( selected on the Screen1 component Properties (default , sans serif , serif or monospace )) if you leave the font setting for the component a default.
Here is an example: the device font is set on the device to choco cooky and
and those components that have the ability to set the font (circled in red) display with choco cooky, the others do not. What happens if you set the component font to
sans serif , serif or monospace in App Inventor while using a custom device font? I do not know. Experiment and let us know.
As far as I know, developers cannot change the device font setting with code (it has to be done manually). You can try to use an intent to change the device's font but since it is a system command, I expect the ActivityStarter cannot be used to do that. You can experiment and let us know Jeff.