title: Theme dark turns textfield to white
body: <p>when I try to use the dark theme, it turns the text field to white instead of a gray background.
  Am I doing something wrong ?</p>
  
  <p>1.Create a text field</p>
  
  <p>2.Set the theme to dark</p>
  
  <pre><code>import React from "react";
  import TextField from "@material-ui/core/TextField";
  import { createMuiTheme } from "@material-ui/core/styles";
  import { ThemeProvider } from "@material-ui/styles";
  
  const theme = createMuiTheme({
    palette: {
      type: "dark"
    }
  });
  
  export default function FilledTextFields() {
    return (
      &lt;ThemeProvider theme={theme}&gt;
        &lt;TextField id="myfilled-name" label="Name" variant="filled" /&gt;
      &lt;/ThemeProvider&gt;
    );
  }
  </code></pre>
  
  <p><a href="https://codesandbox.io/embed/material-demo-18s9j" rel="nofollow noreferrer">https://codesandbox.io/embed/material-demo-18s9j</a></p>
  
  <p>Thanks</p>