How to use with meteor webpack?

Hello everyone.
I’m trying meteor to build a prototype using react+webpack+react-toolbox, according to the react-toolbox docs in order to theme the components i should create a scss file with the overrides and prepend it to every stylesheet using this:

sassLoader: {
  data: '@import "' + path.resolve(__dirname, 'theme/_theme.scss') + '";'

the thing is that meteor webpack uses a webpack.json instead of the regular webpack config file
how should i use that code in the json file ? or should i create anyway a regular webpack.config and add those lines ??

any help will be appreciated