grunt-sass + grunt-contrib-watch

Gruntfile.js

module.exports = function(grunt) {
  grunt.initConfig({
    pkg: grunt.file.readJSON('package.json'),
    sass: {
        options: {
            sourceMap: true
        },
        dist: {
            files: {
                'styles/css/widgets.css': 'styles/sass/widgets.scss'
            }
        }
    },
    watch: {
      sass: {
        files: ['styles/sass/*.scss'],
        tasks: ['sass:dist'],
      },
      livereload: {
        options: { livereload: true },
        files: ['styles/css/*.css', '*.html'],
      },
    },
  });

  grunt.loadNpmTasks('grunt-sass');
  grunt.loadNpmTasks('grunt-contrib-watch');

  grunt.registerTask('default', ['sass:dist', 'watch']);
};