title: Watch specific file changes using webpack
body: <p>Currently I use less-loader along with <code>modifyVars</code> to overwrite some less variables in order to customize the styling.</p>
  
  <pre><code>import theme from './theme.js';
  
  {
      test: /\.(css|less)$/,
      loaders: ['style', 'css?sourceMap', 'postcss', `less-loader?{"sourceMap":true,"modifyVars":${JSON.stringify(theme)}}`],
  }
  </code></pre>
  
  <p>but for every change in <code>theme.js</code> I have to restart the server, (doing npm run ) is there a way to make it automatically just like if I change a less file and it will replaced immediately?</p>
  
  <p>theme.js</p>
  
  <pre><code>module.exports = {
    '@primary-color': '#1DA57A',
    '@font-size-base': '14px',
    '@btn-font-weight': '200',
  };
  </code></pre>