refactor(themes): remove the outer-content class from core (#16589)

BREAKING CHANGES

The outer-content class has been removed in favor of setting the color in your app instead:

```
.outer-content {
  --background: #f2f2f2;
 }
```

core/src/components/thumbnail/test/preview/index.html

@@ -38,12 +38,6 @@
         <ion-label>Wide Thumbnail</ion-label>
       </ion-item>
     </ion-content>
-
-    <style>
-      ion-toolbar {
-        --background: white;
-      }
-    </style>
   </ion-app>
 </body>