How to configure webpack to generate multiple CSS theme files?

I finally found a solution. What I did is generate the css with a variable ($theme)having different values for each generated css file. And in my code, my style depends on the value of $theme.

It is working with (yes, these are old version. Sorry):

"webpack": "2.2.0",
"webpack-combine-loaders": "2.0.3",
"multi-loader": "git://github.com/sshmyg/multi-loader.git#cbaa35f8936a939968adb78301be0204e36f30cd",
"extract-text-webpack-plugin": "git://github.com/ONE-LOGIC/extract-text-webpack-plugin.git#831af7b65ce749069993e60bd9cb51c637d4e98b",

then my webpack config contains this:

const extractDark = new ExtractTextPlugin({ filename: `css/style.dark.css`, allChunks: true });
const extractLight = new ExtractTextPlugin({ filename: `css/style.light.css`, allChunks: true });
...
config.module: {
  rules: [
    {
    test: /\.css$/,
    use: extractDark.extract({
      fallback: 'style-loader',
      use: [
        { loader: 'css-loader', query: { sourceMap: true } },
        { loader: 'postcss-loader' },
      ],
    }),
  }, {
    test: /\.css$/,

    use: extractLight.extract({
      fallbackLoader: 'style-loader',
      use: [
        { loader: 'css-loader', query: { sourceMap: true } },
        { loader: 'postcss-loader' },
      ],
    }),
  },
  {
    test: /\.scss$/,
    use: multi(
      combineLoaders(extractDark.extract({
        fallback: 'style-loader',
        use: [
          {
            loader: 'css-loader',
          }, {
            loader: 'resolve-url-loader',
          }, {
            loader: 'sass-loader',
            options: {
              includePaths: [],
              sourceMap: true,
              data: '$theme: dark;',
            },
          },
        ],
      })),
      combineLoaders(extractLight.extract({
        fallback: 'style-loader',
        use: [
          {
            loader: 'css-loader',
          }, {
            loader: 'resolve-url-loader',
          }, {
            loader: 'sass-loader',
            options: {
              includePaths: [],
              sourceMap: true,
              data: '$theme: light;',
            },
          },
        ],
      }))
    ),
  }
  ]
};
....
config.plugins.push(extractDark, extractLight);

Finally, my stylesheets have code like this:

$backgroundcolor: white;

@if $theme == dark {
  $backgroundcolor: black;
}

.myClass {
  background-color: $backgroundcolor;
}