This repositories is minimal part of core BulmaCSS that contains layout features only.
You can see in package.json file, there is some npm scripts. You can execute each using npm run <script-name>.
Just open your terminal and run npm run build. But, before you can build this, you have to read Preparation part.
As long as you know, this project using npm as main tool, so you should install npm first and before you can build this, you have to install some dev dependencies. It's simple. Just open your terminal and run npm i.