You do not need an account to use the style guide as a reference but creating a Figma account will give you access to exporting assets or duplicate the file to use the prebuilt components.
We use UIkit as our base framework which is compiled with our custom CSS.
SASS is used to keep the CSS organized and easy to change. You can compile this in several ways, based on your preference and familiarity on CLI. Codekit is the easiest way to get started but you can also use Grunt or Gulp if you are familiar with them.