使用下面的代码更改边框颜色。

InputDecorator(
      decoration: const InputDecoration(
        focusedBorder: OutlineInputBorder(
            borderSide: BorderSide(
          color: Colors.red, // Set your desired border color
        )),
        enabledBorder: OutlineInputBorder(
            borderSide: BorderSide(
          color: Colors.red, // Set your desired border color
        )),
      ),
      child: DropdownButtonHideUnderline(
        child: DropdownButton<String>(
          value: selectedValue,
          onChanged: (String? newValue) {
            setState(() {
              selectedValue = newValue;
            });
          },
          items: [
            DropdownMenuItem<String>(
              value: 'Option 1',
              child: Text('Option 1'),
            ),
            DropdownMenuItem<String>(
              value: 'Option 2',
              child: Text('Option 2'),
            ),
            DropdownMenuItem<String>(
              value: 'Option 3',
              child: Text('Option 3'),
            ),
          ],
        ),
      ),
    )

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我可以通过添加inputDecorationTheme来更改边框颜色

DropdownMenu(
  dropdownMenuEntries: [],
    inputDecorationTheme: InputDecorationTheme(
      enabledBorder: OutlineInputBorder(
        borderSide: BorderSide(
          color: Colors.red,
      ),
    ),
  ),
),

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