If you're lucky, the email application uses a template to display its output, in which case you can just update the template to look like the rest of your site.

The more annoying (and more likely) scenario is that the script is just directly printing HTML to the browser. In that case, the best way is to modify it to use some kind of templating system, then create an appropriate template. That way when your site changes in the future, you have an easier job ahead of you. Easier in the short-term would be to just change what it prints to match the rest if your site.

Another thought: There are lots of form mail scripts available; if the script you have is difficult to configure to display how you'd like, it may be easier to switch to one that's easier to configure.

One more thought: Make sure that, whatever you're doing, your form mail script can't be used to send spam. Otherwise pammers will eventually find it and make a horrible, horrible mess.