The stock email application comes pre-installed on your device by the manufacturer. You cannot change the look of the icon, design/look of the application is part of the software. If you want, depending on your email provider you can download a separate email application for free from the Google Play Store.
For example, if you use Gmail.com just utilize the Gmail application that is already on your device to access your emails. If you have Yahoo or Outlook, Live, you can download the Outlook Email application and access your email that way. If your email provider has their own email application you can download that's what I would recommend. The stock email application on Android devices is not very reliable in my experience, download the appropriate application for your email and you'll be a lot happier in the long run. Less errors and things that can go wrong.
