New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  on: false,
  
  boundStyle: Ember.computed('on', function () {
    if (this.get('on')) {
      return Ember.String.htmlSafe('background-color: red;')
    } else {
      return Ember.String.htmlSafe('background-color: blue;')
    }
  }),
  
  actions: {
    toggleOn() {
      this.toggleProperty('on')
    }
  }
});

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

.something {
  width: 50px;
  height: 50px;
  transition: background-color 2s;
}

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}

<div class='something' style='{{boundStyle}}' />

<button {{action 'toggleOn'}}>Toggle</button>
<br>
<br>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}