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/text/test/preview/index.html

@@ -72,10 +72,6 @@
   </ion-app>
 
   <style>
-    ion-toolbar {
-      --background: white;
-    }
-
     .color-purple {
       color: purple;
     }