Fix theme imports for vanilla js (#1742)

* Fix theme imports in `app.js`

* Add note for theme in migrate

docs/app/templates/public-pages/docs/migrate-7-0-to-8-0.hbs

@@ -25,7 +25,11 @@
     <p>Helper: The helper <code>ember-power-select-is-selected</code> was renamed to <code>ember-power-select-is-equal</code>. If you have used in your custom components, you need to replace it</p>
   </li>
   <li>
-    <p>Vanilla JS: If you have used vanilla js you must now import the css in <code>app.js</code> manually like (for other css adding examples see under installation)</p>
+    <p>
+      Vanilla JS: If you have used vanilla js you must now import the css in <code>app.js</code> manually like (for other css adding examples see under installation)
+      <br>
+      If you have used a theme you need to import like described in section <a href="https://ember-power-select.com/cookbook/bootstrap-theme">bootstrap</a> / <a href="https://ember-power-select.com/cookbook/material-theme">material</a>
+    </p>
     <CodeBlock @language="js" @code="import 'ember-power-select/styles';" />
   </li>
   <li>

ember-power-select/themes/bootstrap.js

@@ -0,0 +1 @@
+import '../vendor/ember-power-select-bootstrap.css';

ember-power-select/themes/material.js

@@ -0,0 +1 @@
+import '../vendor/ember-power-select-material.css';