MenuFlyout should have page background color in high contrast

[mcooley]

Describe the bug
In recent versions of WinUI, MenuFlyouts have a gray background in high contrast themes. They should have the page background color.

Steps to reproduce the bug
Add the following XAML to a test app:

<Button Content="Open Flyout">
  <Button.Flyout>
    <MenuFlyout>
      <MenuFlyoutItem Text="Test 1"></MenuFlyoutItem>
      <MenuFlyoutItem Text="Test 2"></MenuFlyoutItem>
    </MenuFlyout>
  </Button.Flyout>
</Button>

Expected behavior
The background of a MenuFlyout should match the user's preferred background color, like it does in Windows.UI.Xaml.

Screenshots

Version Info

NuGet package version:
Microsoft.UI.Xaml 2.3.200213001
Microsoft.UI.Xaml 2.4.0

Windows 10 version	Saw the problem?
Insider Build (19628.mn_release.200508-1414)	Yes
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Mobile
Xbox
Surface Hub
IoT

Additional context
Internal bug: 26080077.

[ranjeshj]

This does look like a bug. We should match the behavior of Windows.UI.Xaml in this case.