title: Different style for DropDownButton and DropDownMenuItem
body: <p>I am creating below registration form in Flutter.</p>
  
  <p><a href="https://i.stack.imgur.com/F7Ns1.png" rel="noreferrer"><img src="https://i.stack.imgur.com/F7Ns1.png" alt="enter image description here"></a></p>
  
  <pre><code>TextStyle white =
      new TextStyle(color: Colors.white, decorationColor: Colors.white);
  TextStyle grey =
      new TextStyle(color: Colors.grey, decorationColor: Colors.white);
  </code></pre>
  
  <p>I want to apply white style to DropDownButton and grey to DropDownMenuItem.
  But, the style of DropDownMenu item is also applied to DDButton.</p>
  
  <p>Also, can I "match_parent" the width of DropDownButton like TextField (as in image)?</p>
  
  <p>Here is the code:</p>
  
  <pre><code>     child: new Center(
        child: new ListView(
          shrinkWrap: true,
          padding: new EdgeInsets.only(left: 24.0, right: 24.0),
          children: &lt;Widget&gt;[
            new ListTile(
              leading: const Icon(
                Icons.language,
                color: Colors.white,
              ),
              title: new DropdownButton(
                items:
                    &lt;String&gt;['India', 'Australia', 'USA'].map((String value) {
                  return new DropdownMenuItem&lt;String&gt;(
                    value: value,
                    child: new Text(value, ),
                  );
                }).toList(),
                value: selected,
                onChanged: (String value) {
                  setState(() {
                    selected = value;
                  });
                },
                style: white,
              ),
            ),
            new ListTile(
              leading: const Icon(Icons.smartphone, color: Colors.white),
              title: new TextField(
                decoration: new InputDecoration(
                    hintText: "Phone Number", hintStyle: white),
                keyboardType: TextInputType.phone,
                style: white,
              ),
            ),...
  </code></pre>