Classname merging

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  classNames: "fancy-border"
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

styles.app.css

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

.fancy-border {
 	border: 4px solid red; 
}

.fancy-bg {
 	background-color: green; 
}

div {
 	width: 100px;
  height: 100px;
}

templates.application.hbs

<h1>Classnames merging test</h1>
<br>
<br>
{{my-component style="width: 100px; height: 100px;" classNames="fancy-bg"}}
<br>
<br>

templates.components.my-component.hbs

{{yield}}

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}