Previously, we looked at creating both a starter theme and a subtheme and, while each has its own benefit, we will often want to have the flexibility to develop on the fly. This means that we do not have to worry about managing a set of files already developed. This may sound contradictory to everything we have heard earlier, but taking an agile approach to theming allows for designers to create rich designs outside the boundaries of Drupal. With the introduction of Twig templates, we pretty much broke the mold on having to architect the layout of Drupal in a specific way. So, gone are the days of telling a designer that we can't implement their ideas.
As we create our custom theme, we will have the freedom to use whatever frontend tools are in the wild combined with the ability to implement both CSS and JS Frameworks using libraries, templates, and custom CSS/JS.