Error: Expected 'styles' to be an array of strings

You are only passing the css files, and not the scss files, to to-string-loader, so require('./material2-app-theme.scss') is not returning a string

Change the scss loader to...

{
    test: /\.scss$/,
    loaders: ['to-string-loader', 'css-loader', 'sass-loader'],
    include: [helpers.root( 'src', 'styles' )]
}

Also make sure that ./material2-app-theme.scss is in the include folder

---

FYI You could easily combine these two loaders into one and simply use...

{
    test: /\.(css|scss)$/,
    loaders: ['to-string-loader', 'css-loader', 'sass-loader']
}

For completing the issue, if you use ExtractTextPlugin, the "webpack.config.js" is:

{
    test: /\.(css|scss)$/,
    loaders: ExtractTextPlugin.extract({
        fallback: "style-loader",
        use: ['css-loader?sourceMap', 'scss-loader?sourceMap']
    })
}

Then I solve issue with:

{
    test: /\.(css|scss)$/,
    loaders: ['to-string-loader'].concat(ExtractTextPlugin.extract({
        fallback: "style-loader",
        use: ['css-loader?sourceMap', 'scss-loader?sourceMap']
    }))
}