Confronted with a heavy-traffic (> 700k sessions/month) multi-site installation and tight deadlines we had to figure out how a single Drupal theme could be used to cover all frontend needs using Sass and a refined development approach.
How do you create your theme Sass/JS/CSS if you have common styles for basic elements but different layouts, structures and contents in your multi-site installation? What should the Sass structure look like? What additional tools to you need?
At the Drupalera we created an approach for Drupal 8 that avoided duplicate or unnecessary CSS and allowed highly parallel development for our frontend developers.