add Code Style inside normal text flutter

You can use code_text_field package.

Example:

class CodeEditor extends StatefulWidget {
  @override
  _CodeEditorState createState() => _CodeEditorState();
}

class _CodeEditorState extends State<CodeEditor> {
  CodeController? _codeController;

  @override
  void initState() {
    super.initState();
    final source = "void main() {n    print("Hello, world!");n}";
    // Instantiate the CodeController
    _codeController = CodeController(
      text: source,
      language: dart,
      theme: monokaiSublimeTheme,
    );
  }

  @override
  void dispose() {
    _codeController?.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return CodeField(
      controller: _codeController!,
      textStyle: TextStyle(fontFamily: 'SourceCode'),
    );
  }
}

Output:

For More customization use different options of CodeController

codeController = CodeController(
  text: source,
  patternMap: {
    r'".*"': TextStyle(color: Colors.yellow),
    r'[a-zA-Z0-9]+(.*)': TextStyle(color: Colors.green),
  },
  stringMap: {
    "void": TextStyle(fontWeight: FontWeight.bold, color: Colors.red),
    "print": TextStyle(fontWeight: FontWeight.bold, color: Colors.blue),
  },
);

Output: