[Bug] Background stays white under dark theme on iOS

[Silic0nS0ldier]

Description

When dark theme is active the app background color stays white, while other elements (such as text) are correctly styled (if potentially difficult to see with the incorrect background).

Steps to Reproduce

1. Create new Xamarin Forms app from Visual Studio template
2. Update Xamarin Forms to 4.8
3. Run app on iOS, with dark theme active

Expected Behavior

All in-box controls styled to match current theme (unless attribute explicitly set).

Actual Behavior

App background (not page, whose components have a transparent background) on iOS is always white, formatted text does not respond to style changes (already raised under #10929, so tangential to issue I'm raising here).

Basic Information

• Version with issue: 4.8.0.1534
• Last known good version: 4.6.0.1141 (4.7 untested)
• IDE: Visual Studio16.7.5 with mobile development workload
• Platform Target Frameworks:
  • iOS: 14.0
  • Android: 9.0
• Nuget Packages:
  • Xamarin.Forms 4.8.0.1534
  • Xamarin.Essentials 1.5.3.2
• Affected Devices: Observed on iPhone (iPhone 8 simulator with iOS 14)

Screenshots

Dark them active

Light theme active (after switching from dark without restarting app)

Light theme active (app restarted)

Dark theme (after switching from light without restarting app)

Reproduction Link

https://github.com/Silic0nS0ldier/DarkThemeIosBug

Workaround

Background can be set on the page, using bindings to respond to changes.

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage
    xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    x:Class="WeedWise.MainPage"
    BackgroundColor="{AppThemeBinding Light=White, Dark=Black}"
>
    ...
</ContentPage>

No doubt other workarounds exist, perhaps one that would address all pages without having to modify their source.