We support a feature in App Lab that allows students to customize the fonts used in their apps. As part of this change, we set Arial as the default font for all apps. If you'd like to customize the fonts in your App Lab project, you can do so in Design Mode. Select the element for which you want to change the font, and choose a new "font family" from the dropdown in the "Properties" tab:

App Lab currently supports 13 fonts, displayed here: