Underscores is a great starter theme for WordPress as it provides lots of flexibility. Recently I’ve starting using Sass, Compass, Susy, and Breakpoint. Bringing these great tools into Underscores was somewhat easy.
I used sassy_s as a starting point for breaking up the style.css into different .scss files. I also had to create a config.rb file where I required Susy and Breakpoint. In style.scss I also had to import Compass, Susy, and Breakpoint.
GitHub Repository of Modernscores is here.
Note: You will probably need to use Bundler because of version conflicts. You will also need to install Sass, Compass, Susy, and Breakpoint if they are not already installed.