Highlight selected item in the dropdown in flutter

You can use selectedItemBuilder parameter in the DropdownButton as follow:

DropdownButton<String>(
      value: dropdownValue,
      onChanged: (String? newValue) {
        setState(() {
          dropdownValue = newValue!;
        });
      },
      style: const TextStyle(color: Colors.blue),
      selectedItemBuilder: (BuildContext context) {
        return options.map((String value) {
          return Text(
            dropdownValue,
            style: const TextStyle(color: Colors.white),
          );
        }).toList();
      },
      items: options.map<DropdownMenuItem<String>>((String value) {
        return DropdownMenuItem<String>(
          value: value,
          child: Text(value),
        );
      }).toList(),
    ),

also, you can change the style of different options using items parameter as is clear in the above example. For more info see this.