Stackoverflow - https://stackoverflow.com/questions/54414168/how-to-write-a-css-conditional-in-ember-for-class-attributes

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  usered: false,
  actions:{
    	changeColor : function(){
        	this.toggleProperty("usered");
      }
  }
});

styles.app.css

.red
{
  color:red;
}
.blue
{
  color: blue;
}

table, tr, td
{
  border:1px solid black;
}

templates.application.hbs

<button {{action "changeColor"}}>Change Color</button>
<div class='{{if usered "red" "blue"}}'>
test text
test text
test text
</div>
<table class='{{if usered "red" "blue"}}'>
	<tr><td>A</td></tr>
  <tr><td>B</td></tr>
  <tr><td>C</td></tr>
</table>

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"
  }
}