You can use one custom font at a time in your embeds. (That font needs to be hosted on your website, Hearken can't host it for you. This means that standalone embeds on modules.wearehearken.com can't use custom fonts.)

To get started, go to your Platform Settings:

Then write the name of the font in the Custom Font Family field. Write the font name as it is written in your HTML/CSS. Like in HTML and CSS, you can add fallback fonts after commas in case your first (or second or third) choice fails to render properly. Read more about fallback fonts here. Click Save at the bottom of the Platform Settings page to save your custom font.

Now you can choose to use that font when you're editing your embeds:

Note that if you change the custom font in your Platform Settings, it will change all embeds using that custom font to your new custom font.