Vue.scss

module.exports = {
    chainWebpack: config => {
        config.entry('theme') // you can add here as much themes as you want
            .add('./src/theme.scss')
            .end();
    },

    css: {
        extract: { 
            filename: '[name].css', // to have a name related to a theme
            chunkFilename: 'css/[name].css' 
        },
        modules: false,
        sourceMap: true
    },
}