Repository: flutter
Issue: MergeableMaterial completely ignores CardTheme.color
Labels: framework,f: material design,has reproducible steps,found in release: 3.3,found in release: 3.5,team-design,triaged-design

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## Steps to Reproduce

<!-- You must include full steps to reproduce so that we can reproduce the problem. -->

1. Create a new Flutter application.
2. Replace the code in `main.dart` with the given below

```dart
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        cardTheme: CardTheme(
          color: Colors.red
        ),
        cardColor: Colors.green,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: Text('Flutter Demo'),
        ),
        body: SingleChildScrollView(
          child: MergeableMaterial(
            children: <MergeableMaterialItem>[
              MaterialSlice(
                key: ValueKey<String>('A'),
                child: SizedBox(
                  width: 100.0,
                  height: 100.0,
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}
```
3. Run the application

**Expected results:** <!-- what did you want to see? -->
The color of the `MaterialSlice` should be red but it's green. It completely ignores the `CardTheme.color` and uses `ThemeData.cardColor`, whereas the first priority should have been given to `CardTheme.color`. Have a look here
https://github.com/flutter/flutter/blob/7c6f9dd2396dfe7deb6fd11edc12c10786490083/packages/flutter/lib/src/material/mergeable_material.dart#L542
https://github.com/flutter/flutter/blob/7c6f9dd2396dfe7deb6fd11edc12c10786490083/packages/flutter/lib/src/material/mergeable_material.dart#L614
**Actual results:** <!-- what did you see? -->
The color of `MaterialSlice` is green.

<img src="https://user-images.githubusercontent.com/34301187/89807319-138e8880-db56-11ea-8927-509189b8e3fa.png" width=300 alt="actual_result_image">

<details>
  <summary>flutter doctor -v</summary>

```
[√] Flutter (Channel master, 1.21.0-8.0.pre.98, on Microsoft Windows [Version 10.0.18363.959], locale en-IN)
    • Flutter version 1.21.0-8.0.pre.98 at C:\flutter
    • Framework revision 77b4505c80 (4 days ago), 2020-08-06 18:51:02 -0700
    • Engine revision cd3ea1e839
    • Dart version 2.10.0 (build 2.10.0-3.0.dev fcafd43f2c)


[√] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at D:\SDK
    • Platform android-29, build-tools 29.0.2
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files (x86)\Google\Chrome\Application\chrome.exe

[√] Visual Studio - develop for Windows (Visual Studio Community 2019 16.7.0)
    • Visual Studio at C:\Program Files (x86)\Microsoft Visual Studio\2019\Community
    • Visual Studio Community 2019 version 16.7.30330.147
    • Windows 10 SDK version 10.0.18362.0

[√] Android Studio (version 4.0)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin version 47.1.2
    • Dart plugin version 193.7361
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)

[√] IntelliJ IDEA Ultimate Edition (version 2019.3)
    • IntelliJ at C:\Program Files\JetBrains\IntelliJ IDEA 2019.3.3
    • Flutter plugin version 45.1.2
    • Dart plugin version 193.7361

[√] VS Code (version 1.47.3)
    • VS Code at C:\Users\Ayush Bherwani\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.13.2

[√] Connected device (5 available)
    • Android SDK built for x86 (mobile) • emulator-5554 • android-x86    • Android 9 (API 28) (emulator)
    • Windows (desktop)                  • windows       • windows-x64    • Microsoft Windows [Version 10.0.18363.959]
    • Web Server (web)                   • web-server    • web-javascript • Flutter Tools
    • Chrome (web)                       • chrome        • web-javascript • Google Chrome 84.0.4147.105
    • Edge (web)                         • edge          • web-javascript • Microsoft Edge 84.0.522.52

• No issues found!
```
</details>