This repo contains SASS partials, which can be seen as the "SumUp-Bootstrap". Include these files in a new web project and you'll already get nice buttons, forms, lists, etc...
And of course: Everything about the Corporate Design is always updated and managed in one place.
The global styles build on Bourbon and it's grid framework companion Neat. So in any project where you want use this, you have to install bourbon and neat for it to work. Please follow installation instructions for rails and non-rails projects in the respective docs.