This App or Theme uses SASS and TypeScript. So when you make changes, they must be converted to CSS and JS - which is done using the packager WebPack. These must be installed to do their work, so to install, do this:

1. Make sure you have node.js installed - or install Node now. This will also install NPM (the node-package-manager) for you.
2. Install all dependencies by running this command: npm ci from the App/Theme folder. 
   In case you're not familiar with node/npm, doing this automatically installs all the parts needed according to the package-lock.json.
   1. If npm ci says it can't run, then try running npm install.
      The difference is that npm ci will will try to install exactly the same versions as we used, but if that information isn't available, npm install will install similar or newer versions using the package.json. 

Verify it Works

In your App or Theme folder, you should now be able to just write webpack