asu theme uses Webpack to compile and bundle SASS and JS.
Make sure you have Node and npm installed. You can read a guide on how to install node here: https://docs.npmjs.com/getting-started/installing-node
If you prefer to use Yarn instead of npm, install Yarn by following the guide here.
Go to the root of asu theme and run the following commands: npm install or yarn install.
Update proxy in webpack.mix.json.
Run the following command to compile Sass and watch for changes: npm run watch or yarn watch.