This is a base theme I built for my own portfolio site at http://richardgilbert.co.
It's a simple Drupal 8 theme, reasonably responsive, making use of viewport units to yield text that should be readable on most any size screen.
Either npm or yarn, in order to install all necessary packages. This theme was built with yarn in mind, but npm should work -- just delete the yarn.lock file, or just ignore it.
Run yarn install or npm install from the root of the theme. It should create a node_modules directory. After it is done, run gulp watch and start working!
You also might want to install the LiveReload extension for your browser (it's available for Chrome and Firefox). Then, while Gulp is running, the site will refresh with every change you make in Sass.