I have tried to add a project font (under File > Project Settings > Font List.
I can add the font family name but I am unable to add the import for the font e.g.
font-family: Roboto, 'Franklin Gothic Medium', Tahoma, sans-serif;
Along with the @Import:
I have included in the projects default.css, but unable to add for project settings.
See if this helps. Fonts (grainge.org)
My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.