Align Acrylic10 tint behavior with official dark/light acrylic palette

[Copilot]

Acrylic10 in QuickLook did not match the issue’s target visual parameters for dark/light mode. This change aligns Acrylic10 tint color and tint opacity with the specified official values while keeping non-Acrylic10 backdrop behavior unchanged.

• Acrylic10 tint now follows official dark/light values

  • Added Acrylic10-specific theme palette in ViewerWindow:
    • Dark: #171717
    • Light: #F2F2F2
  • SystembackdropType.Acrylic10 now uses this palette via GetAcrylic10TintColor().

• Acrylic10 tint opacity set to 0.70

  • Extended WindowHelper.EnableAcrylicBlur(...) with an optional tintOpacity parameter (default remains 0.8 to preserve existing callers).
  • Acrylic10 now calls EnableAcrylicBlur(..., tintOpacity: 0.7).

• No functional drift for other backdrop modes

  • Existing Acrylic/Acrylic11/Mica/Tabbed code paths continue using prior defaults and behavior.
  • Custom WindowBackgroundColor override remains supported and still takes precedence when valid.

case SystembackdropType.Acrylic10:
{
    var acrylicTint = GetAcrylic10TintColor();
    WindowHelper.EnableAcrylicBlur(this, acrylicTint, CurrentTheme == Themes.Dark, Acrylic10TintOpacity);
    break;
}

private Color GetAcrylic10TintColor() =>
    CurrentTheme == Themes.Dark ? Color.FromRgb(0x17, 0x17, 0x17) : Color.FromRgb(0xF2, 0xF2, 0xF2);