customize_dropdown.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  String dropdownvalue = 'Apple';

  var items =  ['Apple','Banana','Grapes','Orange','watermelon','Pineapple'];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text("DropDownList Example"),
      ),
      body: Container(
        decoration: BoxDecoration(
          image: DecorationImage(
            image: const NetworkImage(
                    "https://images.unsplash.com/photo-1579202673506-ca3ce28943ef"
                    ),
            fit: BoxFit.cover,
            colorFilter: ColorFilter.mode( Colors.black.withOpacity(0.4), BlendMode.dstATop),
          )
        ),
        child: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              DropdownButton(
                value: dropdownvalue,
                icon: const Icon(
                  Icons.keyboard_arrow_down,
                  color: Colors.amber,
                ),
                iconSize: 40,
                underline: Container(
                  height: 1,
                  color: Colors.transparent,
                ),
                items:items.map((String items) {
                       return DropdownMenuItem(
                           value: items,
                           child: Container(
                             width: double.infinity,
                             alignment: Alignment.center,
                             padding: const EdgeInsets.fromLTRB(0,8.0,0,6.0),
                             child: Text(items),
                             decoration: const BoxDecoration(
                              border: Border(top:BorderSide(color:Colors.grey,width:1))
                             )
                           )
                           //child: Text(items)
                       );
                  }
                  ).toList(),
                onChanged: (String newValue){ setState(() { dropdownvalue = newValue; });
                },
                isExpanded: true,
                isDense: true,

              ),
            ],
          ),
        ),
      ),
    );
  }
}