On our recent projects we have used Tachyons to build rapidly the user interfaces.
At least for the first few sprints I think we could continue to use Tachyons while we are writing done our thought and ideas about the app's features, however I'm keen to investigate other tools which will help us to let people contribute easily to the UI and to let us maintain and involve the UX rapidly.

I've been keen to use and learn more about grid css (combine with flexbox).
Tailwind which is similar to Tachyons seems also popular at the moment.

Another idea suggested by @nelsonic is to use Elm UI and have a full frontend build only with Elm (see also https://www.youtube.com/watch?v=Ie-gqwSHQr0&t=2s)

We need to make sure that we are able to build a responsive (mobile first) application with the selected tool and that the UI produced will be easily customisable.