title: Change TextField's Underline in Flutter
body: <p>I'm working on an application using Flutter SDK. When I use a <a href="https://docs.flutter.io/flutter/material/TextField-class.html" rel="noreferrer"><code>TextField</code></a> widget, and I focus it, the underline becomes blue. I need to change this color to red, how can I do it?</p>
  <p>Screenshot of what I need to change. I want just the underline to change, <strong>, not the label color.</strong></p>
  <p><img src="https://i.stack.imgur.com/5P06R.png" alt="Screenshot of what I need to change. (I want the underline to change, not the label color)" /></p>