What are the language you are using for your messageBaord.Do you use php ? Personally i dont recommend using pop-up to open a new windowbecause a ot of user will block it . However you could consider using a technique like this http://yensdesign.com/tutorials/popupjquery/ to open up your new page .
You can add a CSS file to your pop_up.html and style it like you would do with any other webpage.
Hope that helped :).