Fonts can easily be added to your instance via the Fonts subsection of the Settings section. There are hundreds of fonts to choose from. All of Google Fonts are available through this interface.
Search for the fonts you'd like to add to your instance. Once you find a font that you like simply click that font's install button.
After installation, fonts will still need to be added to the instance's style sheet. Use the font-family property to add the font(s) to your style sheets.