To use a specific Google Font in the hosted pages (customer portal), just add in the CSS (accessible from the Design section) :
@import url(‘https://fonts.googleapis.com/css?family=NameOfTheFont’);
Example :
@import url(‘https://fonts.googleapis.com/css?family=Raleway’);
@import url(‘https://fonts.googleapis.com/css?family=Poppins’);
If you want to use a font which is not on Google Fonts, just replace the url with the url where the font is accessible. The url of the font must be secured (starting with https).