Unreadable links in RichEditBox with dark theme

### Describe the bug

RichEditBox always displays links using a dark blue (Maybe even simply #0000FF?) color making them unreadable on dark theme.
Manually overriding the color with a light one doesn't seem to work, as light colors seem to be ignored.

### Steps to reproduce the bug

1. Open WinUI 3 Gallery
2. Open "RichEditBox" sample
3. Open a [.rtf file containing a link](https://github.com/user-attachments/files/17542300/Untitled.document.zip)
4. Notice the link being unreadable

### Expected behavior

Links are rendered using a proper foreground by default, and the framework allows choosing any color to be used as link foreground.

### Screenshots


### NuGet package version

None

### Windows version

_No response_

### Additional context

_No response_

Start by reproducing the issue in the WinUI 3 Gallery RichEditBox sample with the linked RTF file, then inspect how link rendering responds to the dark theme and foreground overrides. Done means links use a readable default foreground in dark mode and allow a custom light link color.

Written by the indexing model from the issue text.