1) Click on the Themes button in your account's main navigation
2) Click on the Create New Theme button (or click on one of the themes you've already created)
3) Find the Custom Fonts section and click Enable
4) Find the text area whose fonts / typography you'd like to change. Choose from the fonts in the drop down menu
5) Once you've made all of your font selections, click the Create Theme or Update Theme button.
6) Now, when you create or edit a payment form, you can apply this theme design to the form