Gruntfile.js

grunt.initConfig({
  sass: {
    files: ['<%= yeoman.app %>/styles/{,*/}*.scss'],
    tasks: ['sass:server', 'autoprefixer']
  },
  
  sass: {
    server: {
      files: {
        '.tmp/styles/main.css': '<%= yeoman.app %>/styles/main.scss'
      },
    },
    dist: {
      options: {
        outputStyle: 'compressed',
        sourceComments: 'normal'
      },
      files: {
        '.tmp/styles/main.css': '<%= yeoman.app %>/styles/main.scss'
      },
    },
  },

  concurrent: {
    server: [
      'sass:server',
    ],
    dist: [
      'sass:dist',
    ]
  },

});

package.json

{
  "devDependencies": {
    "grunt-sass": "",
  }
}