Sass / Grunt - D8 theme

Gruntfile.js

module.exports = function (grunt) {
  "use strict";

  require('load-grunt-tasks')(grunt);

  grunt.initConfig({
    pkg: grunt.file.readJSON('package.json'),
    sass: {
      prod: {
        options: {
          outputStyle: 'compressed'
        },
        files: {
          'css/style.css': 'sass/style.scss',
        }
      },
      dev: {
        options: {
          outputStyle: 'expanded',
          sourceMap: true
        },
        files: {
          'css/style.css': 'sass/style.scss',
        }
      }
    },
    postcss: {
      prod: {
        options: {
          processors: [
            require('autoprefixer')({browsers: 'last 2 versions'}), // add vendor prefixes
            require('cssnano')() // minify the result
          ]
        },
        dist: {
          src: 'css/*.css'
        }
      },
      dev: {
        options: {
          processors: [
            require('autoprefixer')({browsers: 'last 2 versions'}), // add vendor prefixes
          ]
        },
        dist: {
          src: 'css/*.css'
        }
      }
    },
    watch: {
      css: {
        files: ['sass/**/*.scss'],
        tasks: ['sass:dev', 'postcss:dev'],
        options: {
          livereload: true
        }
      },
      templates: {
        files: ['templates/**/*.twig'],
        options: {
          livereload: true
        }
      }
    }
  });

  // Default task.
  grunt.registerTask('build', ['sass:prod','postcss:prod']);
  grunt.registerTask('default', ['sass:dev','postcss:dev']);
};

package.json

{
  "name": "drupal-custom-theme",
  "version": "0.1.0",
  "devDependencies": {
    "autoprefixer": "^6.3.6",
    "cssnano": "^3.7.1",
    "grunt": "^1.0.1",
    "grunt-contrib-watch": "^1.0.0",
    "grunt-postcss": "^0.8.0",
    "grunt-sass": "^1.2.0",
    "load-grunt-tasks": "^3.5.2"
  }
}