This has been a tad frustrating! I still can't get it to do it that way I'd like, but I have found a workaround for you:
For some reason, the custom css option that's with the theme won't recognize the import to make PT Sans available; so this leaves two different options (as far as I've been able to find)
1. You can use a custom css plugin to use the @import
2. You could use a child theme to make your changes that way
I prefer to use child themes, but for the sake of testing I just used a custom css plugin and the following:
font-family: 'PT Sans', sans-serif !important;
Hope that helps!