Repository: flutter
Issue: Document when to use BottomNavigationBarThemeData labelStyles
Labels: framework,f: material design,d: api docs,team-design,triaged-design

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## Steps to Reproduce

<!-- Please include full steps to reproduce so that we can reproduce the problem. -->

1. Execute `flutter run` on the code sample <!-- (see "Code sample" section below) -->

**Expected results:** <!-- what did you want to see? -->
I want to see the `selectedLabel.color` red and the `unselectedLabel.color` black
**Actual results:** <!-- what did you see? -->
I see the `selectedLabel.color` blue (default primary color) and the `unselectedLabel.color` grey

<img src="https://user-images.githubusercontent.com/79557484/200126226-425b2d46-6ddf-4c67-8e22-53cc0f51d557.png" width="200" height="400"/>

<details>
<summary>Code sample</summary>

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```
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Test Bottom Theme',
      theme: ThemeData(
          primaryColor: Colors.blue.shade900,
          appBarTheme: AppBarTheme(backgroundColor: Colors.blue.shade900),
          bottomNavigationBarTheme: BottomNavigationBarThemeData(
              unselectedLabelStyle: const TextStyle(color: Colors.black),
              selectedLabelStyle: const TextStyle(color: Colors.red),
              selectedIconTheme: IconThemeData(color: Colors.blue.shade900))),
      home: const HomeScreen(),
    );
  }
}

class HomeScreen extends StatelessWidget {
  const HomeScreen({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(   ),
      body: Container(),
      bottomNavigationBar: BottomNavigationBar(
        items: const <BottomNavigationBarItem>[
          BottomNavigationBarItem(icon: Icon(Icons.home), label: "Home"),
          BottomNavigationBarItem(icon: Icon(Icons.bookmark), label: "Bookmarks"),
        ],
      ),
    );
  }
}

```

</details>

<details>
  <summary>Logs</summary>

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```

~flutter run --verbose

no issues
```

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     Run `flutter analyze` and attach any output of that command below.
     If there are any analysis errors, try resolving them before filing this issue.
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```

~flutter analyze

Analyzing components...                                                 
No issues found! (ran in 3.5s)

```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
~flutter doctor -v

[!] Flutter (Channel unknown, 3.3.3, on Ubuntu 20.04.5 LTS 5.15.0-52-generic, locale en_US.UTF-8)
    ! Flutter version 3.3.3 on channel unknown at /home/bricekk/snap/flutter/common/flutter
    ! Upstream repository unknown
    • Framework revision 18a827f393 (5 weeks ago), 2022-09-28 10:03:14 -0700
    • Engine revision 5c984c26eb
    • Dart version 2.18.2
    • DevTools version 2.15.0

[✓] Android toolchain - develop for Android devices (Android SDK version 33.0.0)
    • Android SDK at /home/bricekk/Android/Sdk
    • Platform android-33, build-tools 33.0.0
    • Java binary at: /snap/android-studio/124/android-studio/jre/bin/java
    • Java version OpenJDK Runtime Environment (build 11.0.13+0-b1751.21-8125866)
    • All Android licenses accepted.

[✓] Chrome - develop for the web
    • Chrome at google-chrome

[✓] Linux toolchain - develop for Linux desktop
    • clang version 10.0.0-4ubuntu1
    • cmake version 3.16.3
    • ninja version 1.10.0
    • pkg-config version 0.29.1

[✓] Android Studio (version 2021.3)
    • Android Studio at /snap/android-studio/124/android-studio
    • Flutter plugin version 70.0.3
    • Dart plugin version 213.7371
    • Java version OpenJDK Runtime Environment (build 11.0.13+0-b1751.21-8125866)

[✓] IntelliJ IDEA Community Edition (version 2022.2)
    • IntelliJ at /snap/intellij-idea-community/390
    • 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

[✓] VS Code
    • VS Code at /snap/code/current
    • Flutter extension can be installed from:
      🔨 https://marketplace.visualstudio.com/items?itemName=Dart-Code.flutter

[✓] Connected device (3 available)
    • sdk gphone x86 (mobile) • emulator-5554 • android-x86    • Android 11 (API 30) (emulator)
    • Linux (desktop)         • linux         • linux-x64      • Ubuntu 20.04.5 LTS 5.15.0-52-generic
    • Chrome (web)            • chrome        • web-javascript • Google Chrome 106.0.5249.119

[✓] HTTP Host Availability
    • All required HTTP hosts are available

! Doctor found issues in 1 category.

```

</details>


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