Today I learned @MonaApp allows you to pick custom-installed fonts for the interface of both their desktop and mobile apps. This has been possible for a while but I only learned about it working in the mobile app today.
Naturally I’ve now changed the settings to show everything with fonts I made myself. (Shown here is @HEX Franklin.)
If you want to do something like this too, you’ll have to use an app like iFont or AnyFont to install the fonts in iOS first (and maybe also restart your device).