As far as I am aware there is no centralized scss repo in Mendix apart from private app store.
I managed to solve this issue by using Mendix private app store (by sharing company Atlas UI resources in private app store and creating starter app that include company Atlas UI). Please have a look at Share Company Atlas UI Resources documentation to find out more details.
Hope this helps!
Here is a start: https://docs.mendix.com/howto/front-end/configuring-your-theme. You might be able to make cssFiles point to a central directory having the theme.