To customize the look and feel of your font (not the font type due to copyright issues), go to the Look & Feel tab on the top from your dashboard:
Then click on General from the left hand sidebar:
And click on the Font tab:
In this panel you can change the text appearance of your Pages, Texts and Title Bar.
You can also customize your fonts in relations to the different App platforms (Android, Apple) and your HTML5 web app: