While you can customize the fonts on your iPhone for some apps with the new AnyFont app, you can’t customize them all. Apps such as Keynote, Pages and Numbers, along with others can be changed to your favorite font though. The app is $1.99 and while it doesn’t include fonts that you need in .tff or .tf format, you can add the fonts from your Mail app. To do this, email whatever font you want to use to your own email address and they tap on the font to download it, and then tap on it one more time to open it.
After you have emailed whatever new font you want to use and have downloaded it, tap on the “Open in AnyFont” option that will pop up. After you do that, the Install Profile section will open in the AnyFont Settings app, and then just tap on the Install button. This will give you a warning, and you can just go ahead and tap on Install again. From that point you will be asked for a passcode that you will have set up when you first opened the app. Type in that passcode and then tap on Done.
Finally, you can open apps such as Keynote and choose your new font.