ShadeEffect crashes with unhandled error with a ThemeResource

### Describe the bug

Assigning a ThemeResource in XAML for the Color property of ShadeEffect throws the following exception:

Exception thrown: 'System.Runtime.InteropServices.COMException' in System.Private.CoreLib.dll
WinRT information: Failed to assign to property 'CommunityToolkit.WinUI.UI.Media.ShadeEffect.Color'. [Line: 316 Position: 48]
An exception of type 'System.Runtime.InteropServices.COMException' occurred in System.Private.CoreLib.dll but was not handled in user code

Using a StaticResource works great.

The following XAML code will reproduce the issue:

```

```

### Regression

_No response_

### Reproducible in sample app?

- [X] This bug can be reproduced in the sample app.

### Steps to reproduce

```text
1. Create a new WinUI application
2. Replace the XAML for MainWindow with the one in the description above
3. Start the application
```

### Expected behavior

Expected theme resource to "just work" the same as a static resource.

### Screenshots

_No response_

### Windows Build Number

- [ ] Windows 10 1809 (Build 17763)
- [ ] Windows 10 1903 (Build 18362)
- [ ] Windows 10 1909 (Build 18363)
- [ ] Windows 10 2004 (Build 19041)
- [ ] Windows 10 20H2 (Build 19042)
- [ ] Windows 10 21H1 (Build 19043)
- [X] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

- [ ] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [X] Windows 10, version 2004 (Build 19041)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

2022

### Visual Studio Build Number

_No response_

### Device form factor

Desktop

### Nuget packages

CommunityToolkit.WinUI.UI.Media 7.1.2

### Additional context

_No response_

### Help us help you

No.