You can create a custom css file, add it to the resources, and then link to it in Settings (head section).
But of course, it would be nice to have those foundation classes built in, like .sticky-shrinknav - whatever.
... or at least be able to edit the custom css file, via CSSEdit or whatever. Linking to resources (in stead of embedding them) and be able to update your resources would accommodate that.
Ontwerpzaam | Papendrecht, The Netherlands
27" iMac i5 3.1GHz / 32GB RAM / 4TB SSHD / 1GB VRAM - macOS 10.12.6 Sierra
Switched from Softpress' Freeway to Coffeecup Responsive Design software