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/checkbox/test/preview/index.html

@@ -80,12 +80,6 @@
         <ion-checkbox slot="end" disabled></ion-checkbox>
       </ion-item>
     </ion-content>
-
-    <style>
-      ion-toolbar {
-        --background: white;
-      }
-    </style>
   </ion-app>
 </body>