I have not been able to add a custom Typekit webfont on platform. I add the Kit Id and it pulls the name of the fonts, however, when applying them to the theme they do not pull the actual font. In this case, the default serif font is shown when selected.

This process is working on Skuid on Salesforce with the same kit Id.

Kit ID: vid7roa

Font Name: Roboto

Platform Example:

SF Example: