title: Is there a way to set a theme for the input decoration prefix icon?
body: <p>I have an input field which has a prefix Icon, but I can't seem to figure out how to set the theme for when the field is not selected. This is what it looks like when not selected</p>
  
  <p><a href="https://i.stack.imgur.com/jUMoZ.png" rel="noreferrer"><img src="https://i.stack.imgur.com/jUMoZ.png" alt="enter image description here"></a></p>
  
  <p>As you can see the icon has a dark color while it should have the light color that the border has. When I select the field I get this</p>
  
  <p><a href="https://i.stack.imgur.com/MDQfE.png" rel="noreferrer"><img src="https://i.stack.imgur.com/MDQfE.png" alt="enter image description here"></a> </p>
  
  <p>Which is much better because now I can actually see the icon. The documentation says</p>
  
  <pre><code>The size and color of the prefix icon is configured automatically using an [IconTheme] and therefore does not need to be explicitly given in the icon widget.
  </code></pre>
  
  <p>But when I set the icon themes</p>
  
  <pre><code>iconTheme: IconThemeData(color: _primaryColorLight),
  primaryIconTheme: IconThemeData(color: _primaryColorLight),
  accentIconTheme: IconThemeData(color: _primaryColorLight),
  </code></pre>
  
  <p>Nothing changes?</p>
  
  <pre><code>  Widget _buildNameField(BuildContext context) {
      final ThemeData theme = Theme.of(context);
      return TextFormField(
        onSaved: (String value) =&gt; _phoneNumber = value,
        keyboardType: TextInputType.text,
        style: theme.textTheme.body1,
        decoration: InputDecoration(
          prefixIcon: Icon(Icons.person),
          hintText: 'Name',
        ),
        validator: (String value) {
          if (value.isEmpty || value.length &lt; 2) return 'Please enter a name with two letters or more';
        },
      );
    }
  </code></pre>
  
  <p>That's my name field. Now I could ofcourse just give the icon a color. But I so happen to be using a package which has a selection menu which also uses a TextFormField and also faces the same problem. So I could ofcourse fork that repo and change the icon color in there aswell. But that seems way to much effort and pretty hacky.</p>
  
  <p>So how can I set it with a theme? </p>
  
  <p><strong>Edit</strong></p>
  
  <p>The documentation says that the TextFormField's prefix icon theme comes from a IconTheme.</p>
  
  <pre><code>return IconTheme(
    data: theme.iconTheme,
    child: ListTileTheme(
      iconColor: theme.iconTheme.color,
      child: MaterialApp(
        home: TheRootPage(),
        theme: theme,
        debugShowCheckedModeBanner: false,
      ),
    ),
  );
  </code></pre>
  
  <p>This works for the ListTileTheme but not for the IconTheme why is this?</p>