Great question @Mathew
You can do that already - select “custom” and then type in the Font name from Foundry or the Theme you are using.
e.g.
“Josefin Slab” (Google Font) found in Kiki Pro theme.
You can see it working here. Josefin selected in theme and typed into Stack:
That said, I’ll release an update today (1.1.5) with an option to select “Theme” and it will auto take the Theme selected font without needing to Type it in 
The custom feature here is also good if using other Stacks on the page and want to tap into that Font.
Update soon
UPDATE:
Has been released!