class property does more then you think

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  classNames: ['my-component'],
  class: ''
});

controllers.application.js

import Ember from 'ember';

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

styles.app.css

.my-component {
  padding: 20px;
}

.my-component div {
  padding: 20px;
}

.my-component.foo1, .my-component.foo2 {
  background-color: red;
}

.foo1, .foo2 {
  background-color: green;
}

templates.application.hbs

<MyComponent @class="foo1" />
<br>
{{my-component class="foo2"}}

templates.components.my-component.hbs

<div class={{class}}>Foobar</div>

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"
  }
}