Repository: flutter
Issue: `PredictiveBackPageTransitionsBuilder` background color not changeable
Labels: platform-android,framework,f: routes,has reproducible steps,P2,team-design,triaged-design,found in release: 3.24,found in release: 3.25

### Steps to reproduce

1. Add a theme to the MaterialApp that includes `PredictiveBackPageTransitionsBuilder` as transition (see code sample).
2. Enable predictive back in developer settings
3. Use predictive back, but don't pop it

=> There's no way of setting the black background to a different color (correct me if I'm wrong)

<img src="https://github.com/user-attachments/assets/f69d4080-2644-4db1-aa8d-260d1e271828" height="720" />

### Code sample

<details open><summary>Code sample</summary>

```dart
pageTransitionsTheme: const PageTransitionsTheme(
  builders: <TargetPlatform, PageTransitionsBuilder>{
    TargetPlatform.android: PredictiveBackPageTransitionsBuilder(),
  },
)
```

</details>


### What target platforms are you seeing this bug on?

Android

### OS/Browser name and version | Device information

Android 14

### Does the problem occur on emulator/simulator as well as on physical devices?

Yes

### Logs

<details open><summary>Logs</summary>

Not needed

</details>


### Flutter Doctor output

<details open><summary>Doctor output</summary>

```console
[√] Flutter (Channel stable, 3.22.2, on Microsoft Windows [Version 10.0.22631.4037], locale de-DE)
    • Flutter version 3.22.2 on channel stable at E:\dev\flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 761747bfc5 (3 months ago), 2024-06-05 22:15:13 +0200
    • Engine revision edd8546116
    • Dart version 3.4.3
    • DevTools version 2.34.3

[√] Windows Version (Installed version of Windows is version 10 or higher)

[√] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
    • Android SDK at C:\Users\jakob\AppData\Local\Android\sdk
    • Platform android-34, build-tools 34.0.0
    • Java binary at: C:\Program Files\Android\Android Studio\jbr\bin\java
    • Java version OpenJDK Runtime Environment (build 17.0.9+0--11185874)
    • All Android licenses accepted.

[√] Visual Studio - develop Windows apps (Visual Studio Community 2022 17.10.6)
    • Visual Studio at C:\Program Files\Microsoft Visual Studio\2022\Community
    • Visual Studio Community 2022 version 17.10.35201.131
    • Windows 10 SDK version 10.0.22621.0

[√] Android Studio (version 2023.2)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin can be installed from:
       https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
       https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 17.0.9+0--11185874)

[√] VS Code (version 1.92.2)
    • VS Code at C:\Users\jakob\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.86.0

[√] Connected device (2 available)
    • Pixel 6 (mobile)  • 25141FDF6001LD • android-arm64 • Android 14 (API 34)
    • Windows (desktop) • windows        • windows-x64   • Microsoft Windows [Version 10.0.22631.4037]

[√] Network resources
    • All expected network resources are available.

• No issues found!
```

</details>