You can use Google Fonts in Showit and if you have a custom font you would like to use, you can use a service like Font Squirrel to create a .woff font file that you can upload to Showit and use for your design. The following video walks you through that process:
Be sure to add your converted .woff file to your media library, just as you would add an image to the media library. Then the custom font will be available on the drop down menu in the design settings.