title: How to add background image on a material ui Dialog component
body: <p>I'm using <code>material-ui version 3.9.3</code> in my React application. I want to add a background image on a dialog. I'm using the <code>Dialog</code> component for it but I'm unable to add a background image on the whole dialog.
  For example:</p>
  
  <pre><code>
      &lt;Dialog
         fullScreen={fullScreen}
          open={this.props.open}
          onClose={this.handleClose}
          aria-labelledby='responsive-dialog-title'
          &gt;
          &lt;DialogTitle
            id='customized-dialog-title'
            onClose={this.handleClose}
            noCloseButtonNeeded={noCloseButtonNeeded}
          &gt;
            {/* some jsx */}
           &lt;/DialogTitle&gt;
          &lt;DialogContent&gt;
            {children}
          &lt;/DialogContent&gt;
        &lt;/Dialog&gt;
  
  </code></pre>
  
  <p>I have tried to add an image using classes and custom CSS but I'm unable to do it. 
  Can anyone help me out to add it? Thanks in advance :)</p>