Toggle Class

components.nav-bar.js

import Ember from 'ember';

export default Ember.Component.extend({
  test: true,
  actions: {
    toggle(){
      this.toggleProperty('test')
    }
  }
})

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
});

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}
.test{
  background-color:red;
}
.not-test{
  background-color:yellow;
}

templates.application.hbs

{{nav-bar}}

templates.components.nav-bar.hbs

<ul class="{{if test 'test' 'not-test'}}">
    <li>Home</li>
    <li>Blog</li>
</ul>
<button {{action 'toggle'}}> Add class </button>

twiddle.json

{
  "version": "0.8.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.5.1",
    "ember-data": "2.5.2",
    "ember-template-compiler": "2.5.1"
  }
}