title: Date/Time Picker Flutter : OK/CANCEL button not visible
body: <p>The date picker widget in flutter displays the ok/cancel buttons in white, and therefore are not visible in a white background.</p>
  
  <pre><code>_displayFromDate = await showDatePicker(
                      context: context,
                      initialDate: now,
                      firstDate: now,
                      lastDate: now.add(new Duration(days: 30)),
                    );
  </code></pre>
  
  <p>I've tried changing the page theme with primary, accent colours as black. Still not visible. The button are there, because I can tap on them, just the titles aren't visible. </p>
  
  <pre><code>[✓] Flutter (Channel master, v1.2.3-pre.67, on Mac OS X 10.13.6 17G5019, locale
      en-GB)
  [✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
  [✓] iOS toolchain - develop for iOS devices (Xcode 10.1)
  [✓] Android Studio (version 3.2)
  [✓] Connected device (2 available)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/iKlLv.png" rel="noreferrer"><img src="https://i.stack.imgur.com/iKlLv.png" alt="enter image description here"></a></p>