First, install Gulp with npm install gulp -g. It's necessary for Semantic UI to build.
Then, cd to main repository folder and type: npm install (some node-gyp errors can show up when installing Angular 2.0, but that's normal apparently). Confirm all default settings during Semantic UI install.
Go to Semantic UI folder with cd semantic and execute gulp build.
In the main repository folder, type npm run lite once installation is over. The app will start on http://localhost:3000.